Hi Grace,

First, should this expression be?

-6-7i+8i^2+4i^3-5i^4

What you need to use here is that $i^2 = -1.$ Using this what is $i^{4} = i^2 \times i^{2}?$ What is $i^{3}?$ Use this to rewrite the expression and you will have an expression with terms that are real numbers and terms that are real numbers times i. Collect term to write the expression in the form $a + bi$ where $a$ and $b$ are real numbers.

Penny