A divided by (a-b) = 2, find (A & sup2; + AB) (a-5b) divided by (a-b) (A & sup2; - 5ab)

A divided by (a-b) = 2, find (A & sup2; + AB) (a-5b) divided by (a-b) (A & sup2; - 5ab)

Why does a = 2a-2b mean a = 2B