I'll write numbers in polar form as Re^{i theta}, meaning magnitude R, angle theta. If you haven't gotten to that notation in class yet (from Euler's Formula), think of it as the polar coordinate pair (R, theta).
-729 has magnitude 729 and angle pi aka 180^circ. The fact that negative reals have angle pi is the true meaning of Euler's Identity, e^{i \pi}=-1.