Hello! Am quite stuck, any help on this one will be apreciated.

4. Use Green's theorem to evaluate the line integral y'dx - x' dy, where (a) C is the circle x2 + y2 = 4 oriented counterclockwise. (b) C is the upper-half of the circle x2 + y2 = 4 from (2, 0) to (-2, 0)