Answer
$2b(a+6)(4a-7)$
Work Step by Step
$ 8a^{2}b+34ab-84b\qquad$...factor out the common term, $2b$.
$=2b(4a^{2}+17a-42)$
... Searching for two factors of $ac=-168$ whose sum is $b=17,$
we find$\qquad 24$ and $-7.$
Rewrite the middle term and factor in pairs:
$=2b(4a^{2}+24a-7a-42)=$
$=2b[4a(a+6)-7(a+6)]$
= $2b(a+6)(4a-7)$
