hey there!
The Gas law is COMBINED GAS LAW
GIVEN :
P1=1.00 atmp
V1=3.50 lit
T1 = 20.0°C = 293K
P2=0.600 atmp
T2=-20.0°C=253K
Now,
Formula =>
P1V1/T1 =P2V2/T2
=> V2 = (P1/P2)(T2/T1)(V1)
=> V2 = (1.00/0.60)(253/293)(3.50)
=>V2 = 5.01403 L
=> V2 = 5L (approx).