Answer
$(x-3)(x-7)$
Work Step by Step
The given expression, $
y^2-10y+21
,$ has $c=
21
$ and $b=
-10
.$
The possible factors of $c$ are $
\{ 1,21 \}
,\{ 3,7 \}
,\{ -1,-21 \}
,\{ -3,-7 \}
$. Among these factors, the pair whose sum is equal to $b$ is $\{
-3,-7
\}.$ Hence, the factored form is
\begin{array}{l}\require{cancel}
(x-3)(x-7)
.\end{array}