9a 3a
--------- = --------
7a - 21 21a - 63
============================
reduce a 3 out of the right side
9a a
---------- = ----------
7a - 21 7a - 21
==================================
9 1 divide both sides by a
--------- = ------------
7a - 21 7a - 21
=================================
cross multiply
9(7a - 21) = 7a - 21 distribute the 9
63a - 189 = 7a - 21 subtract 7a from both sides
63a - 7a - 189 = -21
56a - 189 = -21 add 189 to both sides
56a = -21 + 189
56a = 168 divide both sides by 56
a = 168/56 Reduce the answer
a = 84/28 = 42/19 or 2 4/19