Using ideal gas equation, PV = nRT, and since there is no volume change and amount change, the equation is now P = kT, where k =nR/V. Temperature must be in kelvin
From the given, k = (0.82)/ (21 + 273) = 2.78 x 10^-3
Substituting T = -3.5+273, P = 0.75 atm