WELCOME TO THE CUBE ROOTS OF EIGHT!

HERE'S HOW YOU FIND THEM:

(8+0i) has length (magnitude) 8 and an angle to the x-axis, or modulus, of 0 degrees;
so the root circle has radius 8^1/3 = 2,
the first root is at theta1 = 0/3 = 0 degrees,
and the remaining root is at theta = 0 + n*360/3 = 120, 240 degrees.