JEE Main 2023 Mathematics question on Circles

Points $P(-3, 2)$, $Q(9, 10)$ and $R(a, 4)$ lie on a circle $C$ with $PR$ as its diameter. The tangents to $C$ at the points $Q$ and $R$ intersect at the point $S$. If $S$ lies on the line $2x - ky = 1$, then $k$ is equal to