1/9 + 1/2 = 1/x
Let x = length of time it will take both hoses to work together
Multiply both sides by 18x.
2x + 9x = 18
10x = 18
x = 18/10
x = 9/5
x = 1.8 hours
In other words, if both hoses work together, the pool can be filled in 1 hour and 48 minutes.