The answer is; 700 degrees Centigrade
This is derived from the Boyle's Law, Charles's Law, and Gay-Lussac Law ideal gas equation that is defined by the formulae; P1V1/T1 = P2V2/T2
Remember the P1 = P2 and V2 = 2*V1 in this case
P1*V1/350 = P1*2V1/T2
T2 = (P1*2V1/350)/ (P1*V1)
T2 = 350*2 =700