The cylinder can withstand a maximum pressure of 14.9 atm. So at this pressure, the cylinder explodes and this can be taken as the final pressure.
Given information:
Initial pressure of the gas P1=12 atm
Initial temperature of the gas T1=(27+273)=300 K
Final pressure of the gas P2=14.9 atm
Final temperature of the gas T2=?
From Gay lussac's law, P1T1=P2T2 at constant volume.
⇒T2=P2T1/P1=14.9×30012=372.5 K