What are the equation linking static air temperature with true airspeed & altitude?
Provided that your calculations take place for a standard day, below 11 km altitude the Outside Air Temperature is simply a linear function of altitude. From the wikipedia article for Standard Atmosphere:



So OAT = 15 - 1.98 * Altitude, with OAT in °C and Altitude in 1,000 ft. Airspeed data is superfluous.