Answer
$(x-2y)(5x+4y)$
Work Step by Step
$5x^{2}-6xy-8y^{2}$
... Searching for two factors of $ac=-40$ whose sum is $b=-6,$
we find$\qquad-10$ and $4.$
Rewrite the middle term and factor in pairs:
$=5x^{2}-10xy+4xy-8y^{2}=$
$=5x(x-2y)+4y(x-2y)$
=$(x-2y)(5x+4y)$
