.. or, if you want to do it the easy way -
M = V/a gives a = V/M
a/a0 = sqrt(T/T0) gives T = T0 * (a/a0)^2
so, plugging in the constants -
T = 288.15 * [(499/0.84)/661.5]^2 - 273.15
= - 40.8 C
or somewhere nearby, depending on the exact constant values you choose to use.
Going back to basics avoids carrying round offs etc through the calculations. In general, you will get a "better" answer by holding off using the constants until you get to the final equation.
what is the ISA deviation at fl390 with 499 tas and M.84
Normally we presume -56.5 C above 11km so deviation = 15.7 C degrees