We can easily get the function of g when looking at the graph, but sadly you don't attached any graph in this question.
f(x) = 5x + 15
y = 5x + 15 (f(x) is also equal to y)
0 = 5x + 15 (change the y into 0)
5x = 15
x = 15/3
x = 3 or 0 = x + 3
so basically g(x) = x +3
The answer is C. g(x) x + 3