Chem help (partial pressure)

A 8.50 L cylinder containing S2F2 at a pressure of 3420 torr is connected by a valve to 97.0 L cylinder containing SF6 at 760.0 torr pressure. Calculate the partial pressure (torr) of S2F2 when the valve is opened.

I don't understand where to begin. I'd really appreciate it if someone could explain the steps.

total volume =97.0+8.50=105.5L
P1V1=P2V2
P2=[V1/V2] *P1

final partial pressure of S2F2 [8.50L/105.5L]* 3420torr =275.5Torr

final partial p of SF6 =[97.0L/105.5L]*760.0torr=699 torr

P total = P1 + P2
= 4.5 + 1 = 5.5 atm
P1 = 4.5/ 5.5 = .82 atm