Why won't this problem check out

Solve by substitution or addition-or-subtraction method.
2m=n-3
3m=2n+9

m=3
n=9

but when I check it...
2(3)=9-3
6=6

3(3)=2(9)+9
9=27
... it doesn't come out right. Why?

6m = 3n-9
6m = 4n+18
0 = n + 27
n = -27
m = -15
Check -30 = -27-3 = -30 ..........True
-45 = -54+9 = -45...........True