If we know three parameters, we can find and find them unknown. That's exactly what this formula tells us. If we know, for example, Nt and D, we can find P.
Or if we know p, d and T will find N. Or if we know p, n and T will find D and so on. So let's do an example. Our example states that we want to calculate volume that 0.5 grams of methane occupied at 25 degrees Celsius and one atmospheric pressure. So we have one known, we have two knowns our our pressure.