Express (4-5i) (-5+8i) in the form a+ bi, where a and b are integers and i^2 = -1

You can expand like it is a polynomial and then use i^2 = -1.

We use FOIL to get \((4-5i)(-5+8i) = -20 + 25i + 32i - 40i^2\). Now use i^2 = -1 to get the answer.