Answer
$(x+y)(x-4y)$
Work Step by Step
$x^{2}-3xy-4y^{2}$
... Searching for two factors of $ac=-4$ whose sum is $b=-3,$
we find$\qquad 1$ and $-4.$
Rewrite the middle term and factor in pairs:
$=x^{2}+xy-4xy-4y^{2}=$
$=x(x+y)-4y(x+y)$
= $(x+y)(x-4y)$
