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That should be easy unless someone is trying to complicate it, which is always possible. Speed of sound is the square root of the product of three numbers. Gamma = the ratio of specific heats which is generally taken to be a constant for dry air = 1.4; R a gas law function of 287.05 joules per kilogram per Kelvin and T the absolute temperature. If the true air speed is 499 kts that is 256.71 metres/sec. If you are truly at Mach 0.84 then the speed of sound in the freestream is 256.71/.84 = 305.605 metres/sec. Thus the only thing we don’t now know is the absolute temperature. So try 232.4K for the freestream temperature in the formula a = sqrt[gamma*R*T]. That works. But if it is total air temperature they are asking for then that value is multiplied by a factor of [1 + 0.7M^2] if you accept the value of 1.4 for gamma. If you don’t it gets slightly more complex because the gas law function then depends more upon the specific heat for air at constant pressure rather than the ratio of the value for constant pressure divided by that for constant volume and you could argue that this is affected by the adiabatic efficiency and the corresponding stagnation pressure. But the chances are your questioner is mixing Kelvin and Centigrade and I think at this late time of night that minus 40.75*C is the answer they are looking for.