help

Solve for z:
z^2 = 21 - 20 i

Take the square root of both sides:
z = sqrt(21 - 20 i) or z = -sqrt(21 - 20 i)

21 - 20 i = 25 - 20 i - 4 = 25 - 20 i + 4 i^2 = (-2 i + 5)^2:
z = 5 - 2 i or z = -sqrt(21 - 20 i)

21 - 20 i = 25 - 20 i - 4 = 25 - 20 i + 4 i^2 = (-2 i + 5)^2:
 
z = 5 - 2 i       or       z = -5 - 2 i

Thanks guest. I had forgotten how to do that.

So at the very least you have refreshed my memory.