Addition : Addition is the mathematical operation that describes increasing a number by an amount equal to a second number. The mathematical symbol for addition is the plus sign $+.$ The term addition is also used for a generalization of this basic operation on numbers to functions, vectors and matrices.

Equation : An equation is a mathematical statement in which two expressions are written with an equal sign in between. A solution of an equation is a set of variables that makes the statement a true statement.

Expression : An expression is a succession of constants, variables and mathematical operations that if evaluated for specific values of the variables represents a number.

Linear equation : A linear equation is an equation that is equivalent to the equation $mx+b=0.$ A linear equation with $m\neq 0$ has exactly the solution $x=\frac{-b}{m}$

Multiple : A multiple of a number $x$ is any of the numbers $1x,2x,3x,4x,\ldots.$ For example the multiples of 7 include 7, 14, 21 and 70.

Multiplication : Multiplication is the mathematical operation that is a shorthand for adding the same amount several times. For example $3\cdot 4=4+4+4=12.$

Solution : A solution to an equation is a choice of variables that when substituted into the equation results in a true statement.

Substitution : A substitution is replacing a term or variable with another term or variable. For example when solving a two-dimensional linear equation, one can solve the first equation for one variable and then substitute that expression for that variable in the second variable to get a linear equation only involving one variable.

Subtraction : Subtraction is the mathematical operation that describes decreasing a number by an amount equal to a second number. The mathematical symbol for subtraction is the minus sign $-.$ The term subtraction is also used for a generalization of this basic operation on numbers to functions, vectors and matrices.

Variable : A letter in an expression or function that represents an arbitrary number.

Solve either equation for $x$ or $y$ and substitute the resulting expression into the second equation. You can also multiply one of the equations by some multiple and then add or subtract the resulting equation to/from the other equation trying to generate an equation that has only one of the variables $x$ or $y$ in it. Then solve this linear equation with one variable.

Negative : A number is negative if it is $\lt 0.$

Number : A number $x$ is a mathematical symbol representing a quantity.

Positive : Positive means $\geq 0.$

A negative number divided by a negative number results in a positive number.

Area : The amount of unit squares that is needed to cover a 2-dimensional shape.

Centimeter : A centimeter ($cm$) is $\frac{1}{100}$ of a meter.

Circle : A circle is a two-dimensional shape. The boundary of a circle is a set of points that has the same distance (the radius) from the center of the circle.

Diameter : The diameter of a circle is the length of a line segment from the boundary of a circle through the center of the circle to the boundary on the other side of the circle.

Equation : An equation is a mathematical statement in which two expressions are written with an equal sign in between. A solution of an equation is a set of variables that makes the statement a true statement.

Half : A Half is the number equal to $\frac{1}{2}=0.5.$

Pi : Pi or $\pi$ is an irrational number approximately equal to 3.14 that is defined as the quotient of the circumference and the diameter of a circle.

Radius : In a circle or sphere the radius is the distance between the center and the boundary.

Square centimeter : Square centimeter or $cm^2$ is a unit of measurement for measuring area. 1 square centimeter is the area of a square with side length 1 $cm,$ so 10,000 $cm^2$ are one $m^2.$

Calculate the radius and then use the formula $A=\pi \cdot r^2$ for the area $A$ of a circle with radius $r.$