You could actually find the compositions and thus have something to compare. You haven't shared the list of possible answer choices.
(f+g)(x) = 5x - 3 + x + 4 = 6x + 1
(f-g)(x) = 5x - 3 - x - 4 = 4x - 7
(f*g)(x) = (5x-3)((x+4) = 5x^2 + 20x - 3x - 12 = 5x^2 + 17x - 12
There are also the quotient (f/g)(x) and the compositions f(g(x)) and g(f(x)).
WRite them out.
Then you could arbitrarily select x values, such as 2, 10, etc., subst. them into each composition and determine which output is greatest.
They are inverse functions though to be completely thorough your teacher should have also put g(f(x)) = x as well. Though I can see what your teacher is aiming for at least.
The idea is that whatever the output of g(x) is, it's plugged into f(x) and the initial input is the result. So g(x) takes a step forward and f(x) takes a step back undoing everything g(x) did. Which is exactly what an inverse operation does.
