First, make all of your units the same.
420 mmHg x 1 atm/760 mmHg = 0.553 atm
Then use one of the gas laws to solve.
P1V1=P2V2
Plug in the known values and solve for the one remaining.
(2.15atm)(2.00L)=(0.553atm)(V2)
4.3=0.5533V2
V2=4.3/0.5533
V2=7.77 Liters
