Reverse order of polynomial coefficients of type $left(r-xright)^n$

I have a simple question.

I have a polynomial defined by
$$
left(r-xright)^n = a_0 + a_1 x + … + a_n x^n
$$

Is there a simple expression if I put the coefficients in reverse order?

$$
a_n + a_{n-1} x + … + a_1 x^{n-1} a_0 x^n = ?
$$

If $r = 1$ it is easy but for other $r$ I don’t know how to do it.

Edit solution:
Thanks to the general answer of paul blart math cop, the simple expression with reversed coefficient order is:

$$
x^n left(r-frac{1}{x}right)^n = left(rx-1right)^n
$$