Question 33497
You're on the right track but recognise that while one pipe is filling (positive) the pool the other is emptying (negative) the pool. So if you modify your equation to show that, you'll come out ok.
{{{x/3 - x/9 = 1}}} Simplify.
{{{3x/9 - x/9 = 1}}}
{{{2x/9 = 1}}} Multiply both sides by 9.
{{{2x = 9}}} Finally, divide both sides by 2.
{{{x = 4.5}}}

Answer is: 4.5 hours.