Answer
$3(x-3)(x-7)$
Work Step by Step
Factor out the common term of 3, then factor the resulting trinomial.
$3x^2-30x+63=3(x^2-10x+21)$
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.
$-3+(-7)=-10$ and $-3\times(-7)=21$, so the other two terms are -3 and -7.
