For these questions, it is best to combine three well known equations...

Ideal gas Law (PV = nRT)

P = pressure
V = volume
n = number of moles
R = Ideal gas constant
T = temperature in Kelvin

density = mass/volume (d = m/V)

moles = mass/molarmass (n = m/MM)

Take the Ideal gas law and substitute n = m/MM into it to get....

PV = (m/MM)RT

Now re-arrage the above equation by dividing both sides by volume (V)

P = (m/V)(RT/MM)

Now we can substitute d in for m/V

P = d (RT/MM)

re-arranging should result in

MM = (dRT)/V

Now that we have this equation, we simply plug in our numbers

MM = (0.26 g/L)(0.08206 L atm-1 mol-1 K-1)(293 K)/1 L

(Note that since density is in grams per liter, we use a volume of one liter)