In order to solve this question, we will need to use two very common formulas, the first it will be the Ideal gas Law, but with a few changes, and the second it will be the density formula:
PV = nRT, this is the Ideal gas formula, and we have:
P = 3.00 atm
T = 45°C, but we need it in Kelvin, 318 K
R = 0.082, this is the gas constant
d = m/V, this is the density formula, and we have:
d = 2.65 g/L
Now let's make a few changes on the ideal gas formula, first switching n (number of moles) for m/MM (mass/molar mass)
PV = m/MM * RT
P*MM/RT = m/V, now we have m/V, which is the same as density, now we can use the values
3 * MM/0.082 * 318 = 2.65
3 * MM/26.076 = 2.65
3MM = 69.10
MM = 23.03g/mol, this is the final molar mass