I need help. I suck at doing Math.

Here it goes...

An inlet pipe can fill a swimming pool in 9hrs and an outlet pipe can empty the pool in 12hrs. Through an error, both pipes are left open. How long will it take to fill the pool?

(Algebraic solutions only!)

I tried to set it up, but I'm not sure if I did it right, and my answer was 9hrs.

9/x = 12/x+3

LCD = x(x+3)

9x+27=12x

27=3x

x=9

It seems too easy. I'm doing it wrong, I can feel it.

Your help is appreciated! =)