3. Consider the function f(3:, y) = 3323; + x + y and consider the point P = (2, 2). (a) (5 points) Explain (in a sentence) why f is differentiable at P. (b) (5 points) Compute the gradient V f at the point P. (c) (5 points) Estimate the value of f at the point (2.1, 2.1)