Simply the following expression: 1 / v/18

g = 28,000 - 700t

Step-by-step explanation:

This solution reads, in words,

"the amount of water remaining in the pool is equal to 28,000 gallons minus 700 gallons per hour", which is what your situation is asking you.  You start with 28,000 gallons and are pumping out (subtracting) 700 gallons per hour.

g is what remains