At constant temperature, you are dealing with Boyle's Law (look it up and read about it).
It can be written as P1V1 = P2V2
P1 = initial pressure = 1.56 atm
V1 = initial volume = ?
P2 = final pressure = 6.40 atm
V2 = final volume = 17.0 L
P1V1 = P2V2 and solving for V1 we have...
V1 = P2V2/P1 = (6.40 atm)(17.0 L) / 1.56 atm
V1 = 69.7 L (3 sig. figs.)