This is an ideal gas problem. The gas inside the balloon is considered ideal. Ideal gas equation is a function pressure, temperature, amount and volume. Note: amount is constant since the balloon ins closed. Pressure is maintained constant since the walls are flexible. Ideal gas equation is: PV=nRT. Put all constant in one side and variables in one.
P/nR=T/V. To find the answer to the question equate the constants of both situation
T1/V1=T2/V2
(25+273.15)/3=(x+273.15)/2
x=-74.38 degC
