Answer:
temperature is 829.5 °C 
Step-by-step explanation:
given data 
methane = 3.2 atm
temperature = 21 °C = 21 + 273 = 294 K
increase the pressure = 12 atm
to find out 
what temperature must the gas be heated
solution
we know at constant volume 
pressure is directly proportional to temperature 
so 
we can say
P(initial) / T(initial) = P(final) / T(final) ......................1
here P is pressure and T is temperature
 put the value
3.2 / 294 = 12 / T(final)
T(final) = 12 × 294 / 3.2 
T(final) = 1102.5 K 
 so T(final) = 1102.5 - 273 
T(final) is 829.5 °C