In metres per second, speed of sound is the square root of 401.8 x OAT in Kelvin.
For Kelvin, add 273.17 to °C
For knots, divide metres per second by 0.514.
And remember that your result is in TAS, so you need to make any normal adjustments to CAS.
So, at 28,000ft, ISA +10
OAT~-30.4°C = 242.7K
c = SQRT (401.8 x 242.7) = 312.3 m/s = 607.6kn.TAS
You can use a whizz wheel, etc, but I'll just use a data table:
607.6kn TAS = 607.6 x SQRT (sigma for FL280 sHp) = 607.6 x 0.63534 = 386kn EAS ~~ 386kn IAS
G