Since the factors add to equal -18, we know the two factors must both be negative (two negatives added equals a negative -- two negatives multiplied equals a positive).
FACTORS OF 72 (NEGATIVE):
-1 × -72
-2 × -36
-3 × -24
-4 × -18
-6 × -12
-8 × -9
-6 + -12 = -18
Factors of 72 that add to equal -18: -6 and -12