What is the volume of the balloon at this depth?

(2.3 L) x (1.1 atm / 3.5 atm) x (14 + 273) K / (31 + 273) K = 0.68 L

PV=nRT <<<memorize!!

n is moles, R is gas constant (use correct one!), T is temp in K (C+273.15)

Since you have a before and after with the same n &R simplifies to

PV/T initial = nR = PV/T final

Now sub in your numbers and solve

SI Units R values:

8.31446261815324 J⋅K−1⋅mol−1

8.31446261815324 m3⋅Pa⋅K−1⋅mol−1

8.31446261815324 kg⋅m2·K−1⋅mol−1s−2

8.31446261815324×103 L⋅Pa⋅K−1⋅mol−1

8.31446261815324×10−2 L⋅bar⋅K−1⋅mol−1

0.082057 L atm mol-1K-1

62.36L·torr/mol·K = L·mmHg/mol·K

pV = nRT  -->  note n*R = constant so

Let V0, p0, and T0 be volume, pressure and temperature at surface (respectively) and

V1, p1, T1 be volume, pressure and temperature at depth (respectively)

Using ideal gas equation of state  nR = p0*V0/T0

then using it again for the balloon at depth

p1*V1 = nRT1 = {p0*V0/T0} T1  -->  solve for V1

V1 = {p0*V0/(p1*T0}}*T1

Plug in values and you're finished