[Math] Rate of change of area of a square with respect to side length

Question

I have been asked to find the rate of change of the area of a square with respect to the length of its side when the side is 4ft.

This is how I thought I should do it.

Area=$s^2$

$\frac{d(a)}{d(s)}=2 s$

Now I thought that I could just replace s with 4ft, however the answer is $\frac{8 \text{ft}^2}{\text{ft}}$,

what am I missing?

Answer 1

$8 ft^2/ft = 8 ft$, which is the answer your formula gives. So your work appears to be correct.