Suppose f(3), (not x = 3), is dropped into the g(x) machine. This is written as g( f(3)). What is this output?
Using the same function machines, what is f(g(3))? Be careful! The result is different from the last one because the order in which you use the machines has been switched! With f(g(3)), first you find g(3), then you substitute that answer into the machine named f(x) .
f(3) = 12
so g(f(3)) = g(12) = 121
f(g(3)) = 17