3/5 - 6/9:
3/5 = 3•9 / 5•9 = 27/45
6/9 = 6•5 / 9•5 = 30/45
3/5 - 6/9 = 27/45 - 30/45
= 27-30 / 45
= –3/45
= –1/15
