Calculus question

Question

Suppose that the base of a rectangle is currently 4 cm, and the length of the base is increasing at a rate of 2 cm per minute. The height of the rectangle is currently 14 cm, and the height is decreasing at a rate of  cm per minute. 

At what rate is the area of the rectangle changing at the current moment?

Answer 1

b' = 2 cm/min, b= 4 cm

h' =-1/2 cm/min, h = 14 cm

Area = A =bh = base x height

take the derivative with respect to time, 1st term x derivative of 2nd + 2nd x derivative of 1st

A'(t) = bh' + hb' = 4(-1/2) + 14(2) = -2 + 28= 26 square cm per minute