Inverse Trigonometric Functions

If \(\cos ^{-1} x-\cos ^{-1} \frac{y}{3}=\alpha\), where \(-1 \leq x \leq 1$, $-3 \leq y \leq 3, x \leq \frac{y}{3}\), then for all \(x, y, 9 x^2-6 x y \cos \alpha+y^2\) is equal to