The distance around a round object (i.e. a circular pool, in this case) is also known as its circumference.
The formula for working out circumference is π × d, where π is the value of pi (usually rounded to 3.14) and d is the diameter.
Therefore, to work out the diameter, you just have to work backwards.
So you know the circumference, and you know the value of pi, therefore you just do:
C = π × d
67.51 = 3.14 × d
Now you just have to divide both sides by 3.14 to isolate the value of d and work out the diameter:
67.51 ÷ 3.14 = d
21.5 = d
Therefore, the diameter is 21.5 ft.