The RMS speed of the molecule is given by vRMS=M3RT. Therefore, to double the speed the temperature must become 4 times.
Therefore, the change in the temperature is ΔT=3T0.
Now the process is isochoric (enclosed vessel) therefore, the heat required will be,
H=nCvΔT⇒H=2856×25R×3×(273+127)=12000cal=12kcal