31. Let f(x, y, z) = x² + y2-2z and S be the surface defined by z = 7x-3y+1, -1≤ x ≤ 1, 0 ≤ y ≤2. Determine the surface integral of f over S. 32. Let f(x, y, z) = 2xy-4z and S be the surface with equation z = 5x-3y-3,0≤ x ≤2 and 0 ≤ y ≤1. Evaluate the surface integrals f(x, y, z)dS S 2