#-150^circ# is coterminal to #210^circ#, so locate #210^circ# on your coordinate plane. It's #30^circ# past #180^circ# in the counterclockwise direction. Imagine standing at the origin facing #210^circ#. 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^circ# and walking forward 3 units so another representation of this point is #(3,30^circ)#.
Since we can convert polar to rectangular, we can also find the rectangular point:
#x=r*cos(theta) #
#x=-3cos(210^circ)=-3(-sqrt(3)/2)=(3sqrt(3))/2#
#y=r*sin(theta) #
#y=-3sin(210^circ)=-3(-1/2)=(3)/2#
So you end up at the point #((3sqrt(3))/2,3/2)#
