Hello everyone, I'l start by saying that this is my first time asking a question here and I just recently started using/learning rmarkdown so I hop eyou'll forgive me if I make/made some mistake(s).

My question is, I'd like to put two arrows under the divisors of two fractions in an equation formula, but the method I found for doing so is (I assume) abominable.
'''
$$P_{tot}=\frac{F_{tot}}{S}=\frac{m}{l}N\overline{{V_x}^2}\frac{1}{S}$$
$$\quad\quad\quad; \downarrow\ \quad\quad\quad\quad\quad\quad\quad\quad;:\downarrow$$
$$\quad\quad\quad;; lS=V_{box}$$
'''
Can anyone point me to a method to make this better/more elegant? I tried using the \begin{align} function but it won't allow me to use "&" in the divisor of a fraction.

Thanks for the reply! I didn't intend to put the thing there, it must have been a trascription issue. The colons and semicolons are probably to increase the spacing (: ;). The formula is center-aligned

Yeah, but I meant to ask if there was a way to do this in a way that wouldn't require me to use the 'ol "just put spaces until it looks good" and maybe would do it automatically