−150∘ is coterminal to 210∘, so locate 210∘ on your coordinate plane. It's 30∘ past 180∘ in the counterclockwise direction. Imagine standing at the origin facing 210∘. If r is positive you would walk forward into QIII but since r is negative you're going to walk backward into QI. Walk 3 units backward into QI. This is equivalent to facing 30∘ and walking forward 3 units so another representation of this point is (3,30∘).
Since we can convert polar to rectangular, we can also find the rectangular point:
x=r⋅cos(θ)
x=−3cos(210∘)=−3(−√32)=3√32
y=r⋅sin(θ)
y=−3sin(210∘)=−3(−12)=32
So you end up at the point (3√32,32)
