What would the formula to use be....If the air temperature drops 6.5 Celsius degrees per 1000 m increase in altitude, compute the temperature at the tropopause if the sea-level air temperature is 15 degrees C and the altitude of the tropopause is11km

s = temp at sea-level ... a = temp at altitude

a = s - [(6.5ºC / km) * (km altitude)]

in this case ... a = 15ºC - [(6.5ºC / km) * (11 km)]