Answer:
5x10^3 L
Step-by-step explanation:
Charle's Law states that the volume of a fixed amount of gas maintained at constant pressure is directly proportional to the absolute temperature of the gas, for a constant amount of gas we can write:

As the pressure of the balloon doesn't change, we can use Charle's Law to solve the problem. Firs we change the given temperatures to absolute temperature units ( °K), using the following relations:
°K=273,15+°C
Therefore:
V1=4.6 x 10^3 L, T1=273,15+31=304,15°K
V2=?, T2=273,15+56=329,15°K

The new volume of the balloon is 5x10^3 L.