What is the answer for -8i-7i?

Answer 1

Numbers of the form a + ib (where i = -11/2; a and b are real numbers) are called complex numbers.

For any two complex numbers (a+ib) and (c+id):

Addition:

(a+ib) + (c+id) = (a+c) + i(b+d)

So, -8i-7i = (-8-7)i = -15i