At what pressure does methane (CH₄) have a density of 17.2 g/L at 49.5 °C?

Answer 1

Here you are using the ideal gas law in the format PM = dRT (obtained from PV = (g/M) RT (where d=g/V) gives P=dRT/M

Remembering that the temperature needs to be in Kelvin and R = 0.0821 L.atm/mol.K

Substituting for the values and solving for Pressure (in atm) gives

P = [17.2 g/L * 0.0821 L.atm/mol.K *322.5 K ] / [16.0 g/mol] = 28.5 atm

I suggest that you review all your formats of the gas laws!