To isolate M in the equation it can be rearranged to M = dRT/P
Plugging in values the equation would become
P = 1.00 atm
M = ???
d = 2.45 g/L
R = 0.0821 atm L / mol K
T = 20 C + 273 = 293 K
M = 2.45 g/L (0.0821 atm L /mol K) 293 K / 1.00 atm
The units that do not cancel are g/mol, providing an answer of
I hope this was helpful.