Evaluate the surface integral S F · dS for the given vector field F and the oriented surface S. In other words, find the flux of F across S. For closed surfaces, use the positive (outward) orientation. F(x, y, z) = −xi − yj + z3k, S is the part of the cone z = x2 + y2 between the planes z = 1 and z = 6 with downward orientation