5. (10 points) A function f(x, y) is differentiable and satisfies f (2, 1) = 3, fx(2, 1) = -0.5, fy(2, 1) = 5. Find the equation of the tangent plane to the surface z = f(x, y) at the point P = (2, 1, 3) and use it to find the best linear approximation to f (2.1, 0.98)