Simplify and solve this equation: 4m + 9 + 5m – 12 = 42. a. m = –5 b. m = 4 c. m = 5 d. m = -4

4m + 9 + 5m - 12 = 42
9m + 9 -12 = 42
9m -3 = 42
9m = 42 + 3 
9m = 45 
m= 45/9
m= 5
Answer is C. 

So first separate the expressions and numbers so it will be easier to solve. expressions on the left and numbers on the right or vice versa. (be carefull with the addition and subtraction signs)
4m+9+5m-12=42
5m+4m=42+12-9
9m=45
m=5

Therefore the answer is C. m=5