first treat the left hand side of the equation as a fraction.
1. x/7 - x/9
compute that and it should give you
2x/63 since the LCM of 7 and 9 is 63 and working out the fraction gives you 2x/63
2.now solve for 2x/63= 6
3.multiply by 63 both sides to eliminate the 63 from the left hand side of the equation
4.your answer should be 2x= 378
5.divide by 2 both sides and that solves x= 189
6.check if 189 fulfils the equation