How do I convert a function $f(x)$ to $f(\frac{x}{a})$?

Question

I have a pretty simple problem but I kinda am confused. I have the function

$$f(x)_a = \sqrt{\frac{2}{\pi}} \cdot \frac{Aa}{x^2 + a^2}$$

And especially: $$f(x=0)_a = \sqrt{\frac{2}{\pi}}\cdot \frac{A}{a}$$

I have to plot it so that $\frac{x}{a}$ is on the abscissa - how do I accomplish that?

I thought of substituting $\gamma = \frac{x}{a}$ but then I'd have an x in there again which is zero...

Thanks for your help.
FunkyPeanut