Answer:
The answer is 23.
Explanation:
First you insert 5 into the X place in both equations. 
f(5)=8(5) - 13 and g(5)=5 - 9
Then you would solve as you normally would.
F(5)=40 - 13, F(5)=27.
G(5)=-4
Then you add them together because that is what the question is asking when the say (f+g). 
Your final answer is 23.