The temperature that the balloon will expand to 3.50 l is 463.56 K
calculation
by use of Charles law formula
that is V1/T1 = V2/T2 where;
V1 = 2.25 l
T1= 298 k
V2= 3.50 l
T2=?
make T2 the subject of the formula T2 =(V2 xT1) /v1
T2= (3.50 l x 298 K) /2.25 L = 463.56 K