Hello!
Let's say we have the expression below.
5+7x-17-5x
To combine like terms, we separate our constants from our variables. As you can see, if we are not given a variable value, you cannot combine 7x and 5. They are not like terms. However, you could combine 7x and -5. Note that combine does not always mean to add. If you combine a negative number, you subtract.
First, let's combine the whole numbers. Note that to find our operation, we use the operation before the second number. In this case, we subtract 17.
5+7x-17-5x
-12+7x-5x
Now we combine the variables.
-12+7x-5x
-12+2x
I hope this helps!