The point P(a, b) in the x y-plane is first rotated counterclockwise by 90^{\circ} around the point (1,5) and then reflected about the line y=-x. The image of P after these two transformations is at (-6,3). What is b-a ?
Answer Choices
A. 1
B. 3
C. 5
D. 7
E. 9