Draw a rectangle and label one side L, and the other perpendicular side W. The parallel sides should be equal to L & W. We could care less about the walkway...it's extra information and the constant W means the pool isn't freaky and changing shapes...aka we are leaving theoretical physics out of this ;) ...if it is constant then so is the length
We will use two common equations: A=LW (Area= Length x Width) and P = 2L + 2W, where P is the Perimeter. (we won't actually need the perimeter equation in this problem, but it is always good to remember it.)
Recall, perimeter is the distance around the pool...so if you were to walk around it you would walk around the long part twice and the shorter part twice, hence the 2L + 2W.
We were given two items in this problem to supplement the equations we know...one is the area, A=100 and the other is that the length is twice as long as the width, L=2W. Let's use L=2W in the area equation since we have a numerical value for A and solve for W since we have an equation L in terms of W.
A=LW -> 100=(2W)W ->100=2W2 -> 50=W2 -> √(50) = W
I don't have a calculator on me so I will leave it to you to take the square root of 50. Once you did this you will have a numerical answer for W, simply plug this number into L=2W and you will have the length. You can check this by multiplying the two together and getting 100 (or close to 100 if you use rounded values). Your L & W are in Feet!! (ft). If the problem ask you to find the perimeter (potential test question) then you would to the same process solving for L & W and then plug them into the perimeter equation.
I hope this helps!