Answer
(a) and (c)
Work Step by Step
Please note that if $n$ is any integer, the point $(r, \theta )$ can be represented as$$(r, \theta ) = (r, \theta +2n\pi ) \\ \text{or} \\ (r, \theta )= (-r, \theta +\pi + 2n\pi ).$$So we have$$(-6, 3\pi )= (-6, 3\pi +2(-1)\pi)=(-6, \pi ) \\ (-6, 3\pi )= (-(-6), 3\pi +\pi +2(-1)\pi )=(6, 2\pi ).$$
