Complete ☻☺

z^2 + 9z – 90 = (z – 6)(z +■)

a.) -15
b.) 15
c.) 90
d.) -9

The answer is 15,

In problems like these, the terms that are a number without a variable on one side will always multiply to the number without a variable on the other side.
Hence, -6 times the black box = -90. Divide -90 by -6 and you get positive 15.

Plug it in to check and you get (z-6)(z+15)=(z^2+9z-90).