Answer: Hello!
here we got two functions, f(x) = 2x + 21, and g(x) = x -24, and we want to solve (f+g)(3).
the notation (f + g)(x) refers to the addition of both functions evaluated in the point x.
(f + g)(x) = f(x) + g(x) = (2x + 21) + (x - 24) = (2x + x) + (21 - 24) = 3x - 3
then, when x = 3 we have (f + g)(x) = 3*3 - 3 = 9 - 3 = 6
so the correct answer is the third one.
is the same as replacing x by 3 in both functions, and after that do the sum between them.