Answer: -30.5 °C
Step-by-step explanation:
Lapse rate between sea level and 2,100m is 6.85 °C/1000m.
Therefore at a height of 2,100m, the temperature should have fallen by;
=

= 14.285 °C
Temperature at 2,100 m will therefore be,
= 35 - 14.285
= 20.615 °C
Lapse rate between 2,100m and 3,500m is 11 °C/1000m.
Therefore at a height of 3,500m, the temperature should have fallen by;
=

= 15.4 °C
Temperature at 3,500 m will therefore be,
= 20.615 - 15.4
= 5.215 °C
Lapse rate between 3,500m and 9,000m is assumed to be the normal rate. Normal rate is assumed to be 6.5 °C/1000m
Therefore at a height of 9,000m, the temperature should have fallen by;
=

= 35.75 °C
Temperature at 9,000 m will therefore be,
= 5.215 - 35.75
= -30.535
= -30.5 °C