Please help. I don't know what I did wrong. I need help with #18. Below me is an image of the #18.

Answer:
m = -12
Step-by-step explanation:
everything is ok til the 3rd equation line
-15m - 30 = -12m + 6
-30 - 6 = -12m + 15m
-36 = +3m
-36/3 = m--------> 3 x -12 = - 36
-12 = m