what does W stand for?
i am sorry but i can not understand you last line.
That W is just a miss-type of a random symbol.
Here is what is going on:
$ \begin{align*}\text{Because }|z^3|&=|z|^3 \\|z|^3&=\dfrac{9}{4}|z|\\|z|^2&=\dfrac{9}{4}\\|z| &=\dfrac{\pm 3}{2} \end{align*}$