To find g(0), you can see what happens to f (x) when x = 0. So, if you look at the graph of f (x), you can see that if x=0, f(0) = 3, then

Now, this point represents the result of performing the composition of functions

Where g(x)=3f(x) - 7 and f(x) is function graphed, and evaluating at x = 0, that is
