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.

'''

$$P_{tot}=\frac{F_{tot}}{S}=\frac{m}{l}*N*\overline{{V_x}^2}*\frac{1}{S}$$
$$\quad\quad\quad; \downarrow\ \quad\quad\quad\quad\quad\quad\quad\quad;:\downarrow$$
$$\quad\quad\quad;; l*S=V_{box}$$

'''

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 in advance for your help!