Finding refractive index using Snell's law

I was trying to calculate the refractive index of solutions by using a laser pen. I’ve read that Snell’s law can not be used directly if the laser enters and exits the prism. So since the wave changes medium twice, Snell’s law shall be used once for each time and combined to be able to find the refractive index. Below is what I found from the website (science buddies.org). Could you please explain the steps I shall take to reach Eq 2 from Eq 1?
$$n_1sintheta_1=n_2sin theta_2$$
$$ n = n_{air} frac{sin[0.5(theta_{md} + theta_p)]}{ sin(0.5theta_p) }$$
where $n$ = index of refraction of solution,

$n_{air}$ = index of refraction of air,

$θ_{md}$ = angle of minimum deviation,

$θ_p$ = apex angle of equilateral triangular prism

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