f(x) = -720h + 10080
10080 is the total amount of water in the pool, 720 is the amount of water you lose each hour.
"h" is the number of hours the pool has been draining, and since you know the pool has been draining for 12 hours, you can plug in 12 for "h".
f(x) = -720h + 10080
f(x) = -720(12) + 10080
f(x) = -8640 + 10080
f(x) = 1440
After 12 hours of draining, 1440 is the amount of water left in the pool. (I don't know the units [ex: gallons, etc.])
