You have to use the equation of state for an ideal gas: PV=nRT. Once you find the pressure in part (a), for part (c) use
Assuming that no air escapes during the compression so that nR remains constant. Hence set the initial value of the LHS equal to the final value:
And from what I understood the pressure remains constant so you have:
Now plug in the values and solve for . By the way, you have to convert your tempratures from Celcius to Kelvins.