Chapter 6 - Section 6.5 - Complex Numbers in Polar Form; DeMoivre's Theorem - Exercise Set - Page 767: 2

$$|z|=3$$

We plot the complex number $z=3i$ the same way we plot $(0,3)$, in the rectangular coordinate system. We find the absolute value of the complex number as follows.$$|z|=|a+bi|=\sqrt{a^2+b^2} \\ \Rightarrow \quad |z|=|0+3i|=\sqrt{0^2+3^2}=3$$