Question

A car is driving northwest at v mph across a sloping plain whose height, in feet above sea level, at a point N miles north and E miles east of a city is given by h(N,E) = 4000+50N+100E
a) At what rate is the height above sea level changing with respect to distance in the direction the car is driving?
b) Express the rate of change of the height of the car with respect to time in terms of v.