Answer
$4(x-1)(x-20)$
Work Step by Step
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.
