Speed of sound in air (Laplace's equation) is given by v= root of [(gamma*R*T)/M] where gamma=the ratio of Cp and Cv of the gas, R=universal gas constant, T=temperature, M= molar mass of gas. So speed is related to the square root of temperature. So for speed to be doubled, temperature (kelvin) must be quadrupled. Hence 4 times of 300K= 1200K = 927 degree celsius is the answer. Hope that helps :)