Suppose a gas sample has a density of

3.68 g/L at STP. What is its density at 25 oC
and 19.0 atm?

The modified gas formula is P*molar mass = density*RT. Solve for M (to determine the molar mass), and using that number substitute back into PM = dRT with the new conditions and calculate the new density. There are others ways to do this but this is the easiest way to explain I think.