Initial pressure of a gas= 1200.0 mm Hg
Final pressure of a gas = 850.0 mm Hg
Initial volume of a gas = 85.0 mL
Final volume of a gas = 350.0 mL
Initial temperature of a gas = 90.0 °C = 90 + 273.15 = 363.15 K
Using the combined gas law we have,
PâVâ/Tâ = PâVâ/Tâ
Tâ = PâVâTâ / PâVâ
where Pâ, Vâ, Tâ stands for initial pressure, volume and temperature respectively  and  Pâ, Vâ, Tâ stands for final pressure, volume and temperature respectively
Plugging the given data in the  combined gas law we get,
(1200.0 mm Hg x 85.0 mL) / 363.15 K = (850.0 mm Hg x 350.0 mL)/ Tâ
Tâ = (850.0 mm Hg x 350.0 mL x 363.15 K)/ (1200.0 mm Hg x 85.0 mL)
Tâ = 1059.2 K = 1059.18 - 273.15 = 786 °C
Final temperature of a gas = 786 °C
To convert °C to  Fahrenheit
(°C à 9/5) + 32
(786°C à 9/5) + 32 = 1446.8 °F
Final temperature in Fahrenheit = 1446.8 °F