That '5' in f(5x) will compress the graph of x^2 horizontally.
If you were to graph f(x) = x^2, you'd get a parabolic graph; the parabola will open UP.
Suppose you graphed f(x) = x^2 on the interval [-4, 4].
Then the graph of g(x) = f(5x) would be graphed on the interval [-4/5, 4/5]. In other words, the graph would be on a shorter interval, one shorter by a factor of 5.
