15 ft 7 inches
Here are the steps to doing this,
1. Write out
6 ft 10 inches
+ 8 ft 9 inches
-------------------------
2. Add the inches first, 10 + 9 = 19
3. Next, add the feet,
6 + 8 = 14
What you have is 14 feet 19 inches
Hopefully, you know that 12 inches is equivalent to one foot.
4. Subtract 12 from 19, and you get 7.
5. Your answer is 15 feet 7 inches
Hope this helps!