If the pool has a diameter of 35 feet, you can use the formula [tex] \pi [/tex] times diameter to get the circumference, which is like perimeter, but for circles.
In this case, the circumference is 109.9, which I'll round up to 110 because the fencing is sold by the foot.
The last step is to multiply 12.5 by 110, which gives you a cost of $1375.