need help please do step by step?

Consider SO₂ at STP :
Pressure, P = 1 atm
Molar mass, M = (32.1 + 16.0×2) g/mol = 64.1 g/mol
Density, d = ? g/L
Gas constant, R = 0.0821 L atm / (mol K)
Absolute temperature, T = 273 K
Mass, m = ? g

For a fixed amount of gas :
PV = nRT
PV = (m/M)RT
PM = (m/V)RT
PM = dRT
d = PM/(RT)

Density, d = 1 × 64.1 / (0.0821 × 273) g/L = 2.86 g/L