well, you know that r cos (theta) is the same as x, so:
a) is simply x = 4 and that is a line.
With b), multiply either side by r, so:
r^2 = 6 r sin (theta), then we know that r sin (theta) is the same as y:
r^2 = 6y, and we also know r^2 = x^2 + y^2, therefore:
x^2 +y^2 = 6y, that is rectangular...
So:
a) x = 4
b) x^2 +y^2 - 6y = 0
Actually, for b you could always complete the square...
x^2 + y^2 - 6y + 9 = 9
x^2 + (y - 3)^2 = 9
This would make a circle
