Simplify and solve this equation: 4m+9+5m-12=42

First you combine like variables
9m - 3 = 42
then add both sides
9m = 45
then divide whats left
m = 5 

You first must add together the same values
So
4m+5m=9m
+9+-12 =-3
So you have
9m-3=42
Then you put the -3 on the other side you you add 3 in to both sides so now you have
9m =45
And then divide 45 divided by 9 = 5
So the answer is m=5