Is the $20,000 supposed to be 20,000 gallons???
If so,
20,000-500x=6,000
would be your equation needed to solve this.
Subtract your 20,000 from both sides.
This should leave you with a new equation of
-500x=-14,000
Divide both sides by -500 to leave a single x variable on one side of the equation. You should be left with a new equation of
x=28
That's it!
It takes 28 days for the water level to reach 6,000.
