To find the function after (f+g)(x) has been performed, add the two functions together since whenever (f+g)(x) is being asked to be performed on two functions, it is asking for the two functions to be added. When you do this, set up your expression like this:
(-3x-5)+(4x-2)
When you have done that, combine your like terms and you should result in the answer x-7 as your new function.