(12 5/13 + x) -9 9/13 = 7 7/13
( 12 5/13 + X ) - 9 9/13 = 7 7/13 12 5/13 + X = 7 7/13 + 9 9/13 12 5/13 + X = 16 16/13 X = 16 16/13 - 12 5/13 X = 4 11/13