(a-b)^7 •(b-a)^5 •(a-b)^3
= (a-b)^7 •{-(a-b)}^5 •(a-b)^3
= -(a-b)^7 •(a-b)^5 •(a-b)^3
= -(a-b)^(7 +5+3)
= - (a-b)^15