3. Consider the function / (x, D) = 632y + 2y - 672 - 6y. (a) Find the first order partial derivatives of f (b) Find the critical points of f. There are four. ( Note you need both the r- and y-coordinates for each critical point.) (c) Find the second order partial derivatives of f. (There are four.) (d) Classify each of the critical points as local maximum, local minimum, saddle point, or cannot be determined