Let D = {(s, t) | s² + t² ≤ 1, s ≥ 0, t ≥ 0} and let X: D → R³ be defined by X(s, t) = (2s + 2t, 2s − 2t, st).

Determine ∫∫ₓ f dS, where f (x, y, z) = 9.