There are two ways to do this.
The first way is to algebraically find (f+g)(x) first and plug in x = 5 later. Doing that method leads us to
(f+g)(x) = f(x) + g(x)
(f+g)(x) = 6x+3 + x-4
(f+g)(x) = 7x-1
(f+g)(5) = 7(5)-1
(f+g)(5) = 34
OR
you can compute f(5) and g(5) first, then add up those sub-results to get
f(x) = 6x+3
f(5) = 6(5)+3
f(5) = 33
g(x) = x-4
g(5) = 5-4
g(5) = 1
Adding up these results gives: (f+g)(5) = f(5) + g(5) = 33+1 = 34
Either way, the final answer is 34