We will assume helium to behave as an ideal gas and apply the ideal gas law:
PV = nRT
For pressure measured in atmospheres and volume measured in liters, the value of the molar gas constant is 0.082. Therefore:
T = PV / nR
T = (2.57 x 15.5) / (1.2 x 0.082)
T = 404.8 Kelvin