At what temperature root mean sqaure of `N_(2)` gas is equal to that of propance gas at `STP` conditions.

At what temperature is the root mean square speed of a an atom is an argon gas cylinder equal to the rms speed of helium gas at - 20^(@)C ? (atomic mass of Ar = 39.9 u and of He = 4.0u)

At what temperature is the root mean square speed of an atom in an argon gas cylinder equal to the rms speed of a helium gas atom at - 20 ^(@)C ? (atomic mass of Ar = 39.9 u, of He = 4.0 u).