The modulus is $\displaystyle \sqrt {x^2 + y^2}$ which here is just $\displaystyle \sqrt{64}$ = $\displaystyle 8$

And as the argument is defined by $\displaystyle tan^{-1} ({|\frac{y}{x}|)}$, and $\displaystyle x$ in this case is zero, you would be basically calculating the inverse tan of $\displaystyle \frac{8}{0}$, and it's impossible to divide through by zero! How, then, is it possible to find the argument of $\displaystyle 8i$?