So = what, in terms of ?
Now, there's a foot wide path all the way around the pool. So the lengths of the sides of the whole rectangular plot are feet and (what)? feet.
Now multiply these two lengths to get the area of the whole plot. This is the function you need.
(b) You'll probably use calculus to find the value of for which this function is a mimimum. (Or you could get an approximate value by drawing a graph.)
Can you see how to do it now?