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.#
#-729 = 729 \ e^{i pi}#
That's #(729, 180^circ)# if that's how you write polar coordinates.