Root mean Square velocity is given by :- 

v (rms)  = sq.rt ( 3P/rho )

Where v (rms) is the root mean square velocity , P is pressure exerted by gas and rho is the density of the gas. 

Now Pressure can be written as P=E*(2/3) where E is the kinetic energy of molecules of gas. 

So you can find pressure and subsitute in the given formula. Now density = mass/volume. So in the question some information about density or mass is missing without which v(rms) cannot be found out. So please mention in the comment section down below.