I would suggest first opening the brackets up.

-(m+4) would became -m-4

-3(7-m) would became -21+3m

and then you add them all up (in this case you would be subtracting because of the negative signs).

You now have -m-4-21+3m = 2m-25 which should be your answer. 

When you are told to "combine like terms," you are asked to add all the same variables, then to add all the constants ("free numbers"). This is a process we use to simplify equations. 

To start, look at what's in the parentheses. -(m+4) This says, "the opposite value of m+4" or "-1 times (m+4)

Solve that (using the distributive property): -(m+4)= -m-4

The next part says -3(7-m) which says, "negative 3 times (7-m)".

Solve that using the distributive property: -3(7-m)= -21+3m (Remember that a negative # times a negative # equals a positive #)

Now your new equation says: -m-4-21+3m 

Now, combine like terms, meaning, add the variables (m's): -m+3m= 2m

Then combine the constants (free numbers) -4-21= -25

New equation should say: 2m-25. This is your answer. 

Just remember, when you need to "combine like terms," you're taking the variables (letters) and adding them together. Then add the constants (free numbers) together. 

Work with the parenthesis first: -(m+4)=-m-4 and -3(7-m)= -21+3m, then combine the like terms, 

-m-4-21+3m= 2m-25 this will be your final answer