(a) Sketch the graph of the function f (x, y) = 9(x 1)2 + yz, and describe this surface. Your sketch must be by hand, and not computer generated. (b) Let x+y+1 g(x,y)= , x-y where x and y are real variables. (i) Find the natural domain D of g, and compute the corresponding range of g. (ii) Sketch, on a single x y-plane, the level curves of g(x, y) of heights z = -1 , z = O, and z = 1. Clearly label the three level curves on your sketch