5. (10 points) A function f(x, y) is differentiable and satisfies f(2, 1) = 3, fr(2, 1) = -0.5, f,(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).