3x+2y=m+3,2x-y=2m-1的解为相反数即x=-y
3(-y)+2y=m+3
-y=m+3
y=-m-3
2(-y)-y=2m-1
-3y=2m-1
y=(1-2m)/3
-m-3=(1-2m)/3
-3m-9=1-2m
m=-10