3. A student was asked to calculate the surface integral of f(x, y, z) = 3xy over the plane z = 5 -x - y in the first octant. Their work is below but they made a mistake. Circle where the mistake happened and explain why it's wrong. J J f (x,y, = ) ds - JJ. f(z,y, 5 - 2 - y)V2 dA (5 -y - V2 / j . 3xy dxdy = V2 3(5 - 3)zy dy Jo 2 3V2 .5 = 2 25y - 10g2 + 3 dy 625V/2 8