1. Determine

2. Use Sine rule to determine (or )

3. Use the tangens function to get the length of h:

4. Don't forget to add the 1.2 km above sea level if you want to get the height of the summit above sea level too.

EDIT:According to Wikipedia Mt. Robson has a height of 3954 m above sea level. If you follow my hints you'll get a height of 3952.66 m, that means 3953 m. Seems accurate enough.