6/4 +2/3 = 3/4 +5/7 - 6/8 =

6/4 + 2/3 = 18+8/12 = 26/12 = 2 2/12
3/4 + 5/7 - 6/8 = 42+40-42/56= 40/56

6/4 + 2/3 = 6*3/4*3 +2*4/3*4 = 18/12 + 8/12 = 26/12 = 13/6 = 2 1/6

3/4 +5/7 -6/8 = 3*14/4*14 + 5*8/7*8 - 6*7/8*7 = 42/56 + 40/56 - 42/56 = 40/56 = 5/7