The formula for the area of a rectangle is:
Area = length × width = L × W
We know the width of the pool, but not it's length. However, we also know the perimeter, which is the distance around a shape. For a rectangle, the perimeter is:
Perimeter = 2L + 2W
From this formula and the given values for width and perimeter, we may solve for L, and the Area of the pool afterwards.
So for a rectangle with P = 132 feet and W = 21 feet, L will be:
132 = 2L + 2 × 21
132 = 2L + 42
132 - 42 = 2L
90 = 2L
L = 45
Now that we know the length, we may proceed solving for the area of the pool:
Area = 45 × 21 = 945 square feet