Your equation would look like this
W= 30t+500
Because the pool is being filled at a rate of 30 liters per minute. You would add 500 because that is what is in the pool to begin with. The first point on your graph should be at (0,500) because at 0 minutes, you have 500 liters of water in the pool. Then at every minute, go up 30 liters.
