已知方程组3x-5y=2m 2x+7y=7m+34的解x,y互为相反数,求m的值怎么求?

3x-5y=2m (1)2x+7y=7m+34(2)(1)*(5/31)+(2)*(8/31),得,（5/31）*(3x-5y) +(8/31)*(2x+7y)=2m*(5/31)+(8/31)*(7m+34),即,x+y=（66/31）m+272/31,又x+y=0,所以（66/31）m+272/31=0,解得,m=136/33