Assuming STP conditions and 22.4 L = 1 mole of gas, you can relate the following:
density = g/L x 22.4 L/mole = g/mole
0.791 g/L x 22.4 L/mole = 17.72 g/mole = 17.7 g/mole (3 sig. figs.)
Doing it the long way and using the ideal gas law: PV = nRT and assume standard temperature and pressure (since these variables aren't given in the question), then you have ...
P = 1 atm
V = 1 L
n = ? moles
R = 0.0821 Latm/Kmol
T = 273K
Solving for n (moles), we have...
n = PV/RT = (1 atm)(1 L)/(0.0821 Latm/Kmol)(273K)
n = 0.04462 moles
Since the 1 L contains 0.791 g, we now have...
0.791 g/0.04462 moles = 17.727 g/mole = 17.7 g/mole (to 3 significant figures)
The answer key of p x V is actually ρ x V where ρ is density or mass/volume, so you have
mass/volume x volume = mass (not molar mass). Not really sure what your question is regarding the ρ and V, but hopefully I've explained this sufficiently. If you have further questions, post them in the comments sections.