How do you combine 4/(3x) - 5/(6x) + 6/(9x)?

We must find the lowest common denominator, which is $9 x$.
$\frac{3 \cdot 4 - 2 \cdot 5 + 1 \cdot 6}{9 x} = \frac{12 - 10 + 6}{9 x} = \frac{8}{9 x}$