a= 2/5 - 5/7 * 14/15
a= 2/5 - 70/105
a= 2/5 - 14*5/21*5
a= 2/5 - 14/21
a= 2/5 - 2/3
a= 2*3/5*3 - 2*5/3*5
a= 6/15 - 10/15
a= -4/15
