Given the function of two variables f(x, y) = 912 + 4y (a) (6 points) what is the domain of f(x, y), what is the range of f(r, y), and are they bounded or unbounded? (b) (6 points) Sketch three level curves of f(r, y) in the ry-plane, and use these to sketch the surface z - 9x2 + 4y in (x, y, z) space