A You can see that $(x-5)(x-4)=0$ for $x=4$ and $x=5$, but there might be other solutions. B Just put $x=4$ and $x=5$ into the equation and calculate. C Because $0\cdot 1=0$ and $-1\cdot 0=0$. D A product of two factors is exactly zero if one of the factors is zero. So (x-5)(x-4)=0 if and only if either (x-5)=0 or (x-4)=0. (x-5)=0 has the solution x=5 and (x-4)=0 has the solution x=4.