How do we rewrite 7/a - 9/b so that both terms have the denominator ab?
7 / a Multiply Numerator and Denominator by b
7 / a = 7 b / a b
9 / b Multiply Numerator and Denominator by a
9 / b = 9 a / a b
7 / a - 9 / b = 7 b / a b - 9 a / a b =
( 7 b - 9 a ) / a b