5/6-12/21+15/7
=5/6-12/21+45/21
=5/6-33/21
=(5×21-33×5)/126
=-60/126
=-10/21
5/6-12/21+15/7=5/6-4/7+15/7=5/6+(15/7-4/7)=5/6+11/7=35/42+66/42=(35+66)/42=101/42
