AMC 10 2021b Problem 9

The point $P(a,b)$ in the $xy$-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 $b - a ?$ $\textbf{(A)} ~1 \textbf{(B)} ~3 \textbf{(C)} ~5 \textbf{(D)} ~7 \textbf{(E)} ~9$