Hello!
The answer is:

Why?
To solve the problem, we must remember how to add/subtract like terms. The like terms are the terms that share the same variable and the same exponent.
For example:

We were able to add only the linear terms (terms that contains the variable x") because they were like terms: they share the same variable and the same exponent.
We need to perform the following operation between polynomials:

So, calculating we have:

Have a nice day!