RMS velocity is proportional to the square root of the absolute temperature ($C_{rms} ∝ \sqrt{T}$). To double the velocity, the temperature must be quadrupled. Initial temp T₁ = 27 + 273 = 300K. New temp T₂ = 4 * 300K = 1200K. In Celsius, T₂ = 1200 - 273 = 927°C.