Chapter 6 - Section 6.5 - Complex Numbers in Polar Form; DeMoivre's Theorem - Exercise Set - Page 767: 9

$$|z|=5$$

We plot the complex number $z=-3+4i$ the same way we plot $(-3,4)$ in the rectangular coordinate system. We find the absolute value of the complex number as follows.$$|z|=|a+bi|=\sqrt{a^2+b^2} \\ \Rightarrow \quad |z|=|-3+4i|=\sqrt{(-3)^2+4^2}=5$$