How would I write -4i in polar form? I know that i solve for y..and I get for but I'm not sure where to go from there.
For any complex number $\displaystyle a + bi$, it can be expressed in the form $\displaystyle \mbox{r cis} \,\,\theta$.
$\displaystyle r = \sqrt{a^2 + b^2}$
$\displaystyle \theta = \arctan{\left(\frac{b}{a}\right)}$
For your question however you should be able to see the number lies on the im axis and so the angle is $\displaystyle \frac{-\pi}{2}$ and there is no real part so $\displaystyle r = 4$