Write down the equation of the plane tangent to the surface

Question

Write down the equation of the plane tangent to the surface z = f(x, y) = y³x^1/2 above a general point (a, b).

Find the differential of the function f(x, y) = e^{-x^2+1} at, the point (1, 4) and use it to estimate the variation in f(x, y) as x increases by 0.01 and y decreases by 0.02.