Elementary Technical Mathematics

$4(x-1)(x-20)$
First factor out the 4, then factor the resulting trinomial. $4x^2-84x+80=4(x^2-21x+20)$ The first term in each binomial must be x, since $x^2$ has no coefficient. The operation in each binomial must be subtraction, since the trinomial has a positive and a negative term. $-1+(-20)=-21$ and $-1\times(-20)=20$, so the other two terms are -1 and -20.