Is it asking you to solve for M? Because, if it is just asking for you to simplify, it should either give you the value for M or make it into an unsimplified equation.
You would do what happened last to the equation(Not counting M,) in this case would be + 21. Since you would do - 21 to get 0, you would subtract 21 on both sides, which gets you positive M = 12.