Define f x y a ax2 - 2x y2 - 4ay where a is a parameter

Question

Question: Define f (x, y, a) = ax² - 2x + y² - 4ay, where a is a parameter. For each fixed a ≠ 0, find the point (x∗(a), y∗(a)) that makes the function f stationary w.r.t. (x, y). Find also the value function f ∗(a) = f(x∗(a), y∗(a), a), and verify the envelope theorem in this case.