I'm having such a hard time with this one :/

A model for the surface area of a human body is given by $\displaystyle S = 0.1091w^{0.425}h^{0.725}$, wherewis the weight (in pounds),his the height (in inches), and S is measured in square feet. If the errors in measurement ofwandhare at most 2%, use differentials to estimate the maximum percentage error in the calculated surface area.

The book's answer is 2.3%