9. Point $P(a, b)$ on the coordinate plane is first rotated $90^{\circ}$ counterclockwise around the point $(1,5)$, and then reflected over the line $y=-x$. After these two transformations, the resulting point is $P^{\prime}(-6,3)$. The value of $b-a$ is ( ).
(A) 1
(B) 3
(C) 5
(D) 7
(E) 9
9. D.

Reverse thinking.
Notice that, the point $P^{\prime}(-6,3)$ symmetric to $y=-x$ is $P^{\prime \prime}(-3,6)$. $P^{\prime \prime}(-3,6)$, when rotated $90^{\circ}$ clockwise about the point $(1,5)$, gives the point $P(2,9)$.
Thus, $a=2, b=9 \Rightarrow b-a=7$.