The definition of sum of the two functions is (f + g) (x) = f(x) + g(x).

(f + g) (x) =

(f + g) (x) =                         (Simplify)

(f + g) (3) =                     (Substitute x for 3)

(f + g) (3) = 14                             (Simplify)

The value of (f + g) (3) is 14.