2x-y=8m (1)
3x+4y=7m (2)
3x-7y=34 (3)
(2)-(3)得11y=34+7m (4)
（1）*1.5得3x-1.5y=12m ( 5)
(5)-(2)得 -5.5y=5m (6)
(6)*2得 -11y=10m (7)
(4)+（7）得34+17m=0
17m=-34
m=-2