When the tubes are connected, pressures become the same.
Also, the total number of moles before and after the connection are the same.
n1=4V300R,n2=16V600R,
total moles = n1+n2=V25R
n1=n2=2n=V25R
n=V50R
When the tube is connected:
P1=P2 =P
PV=nRT
P=nRTV ; (n=V50R )
So, P = V50R×300RV=6 atm