Answer
(a) and (d)
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$$(-2, \frac{7\pi }{6})= (-2, \frac{7\pi }{6}+2(-1)\pi)=(-2, -\frac{5\pi }{6}) \\ (-2,
\frac{7\pi }{6})= (-(-2), \frac{7\pi }{6}+\pi +2(-1)\pi )=(2, \frac{\pi }{6}).$$
