Question

two factors of 72 whose sum is -18

Answer 1

-6 and -12

I'm assuming you're attempting to factor something.
Proof:
-6-12=-18
-6*-12=72

Answer 2

(-3) x (-12) = 72
(-6) + (-12) = -18

so answer
Two factors of 72 whose sum is -18: 
-6 and -12