Question 757949
An intriguing problem.
The slope of the ceiling.
As the wall is 8ft high at the West wall
then if 3ft from the West wall the ceiling has risen by 
2.5 ft (10.5 - 8) then using tan ratio
Tan x = 2.5/3 = 39.8 degrees
The height of the ceiling at the East wall
Distance between West and East walls = 17ft
Using the tan ratio again
Tan = Opposite/Adjacent
Tan 39.8 = Opposite/17 ft
Opposite = tan 39.8 * 17 = 14.2ft
Now you must add this to the height of the
ceiling at the West wall. So height of East wall
ceiling = 14.2 ft + 8 ft = 22.2ft.
Installing a light as far as you can from 
West wall. With a limit of 21 foot stepladder.
First you must take off the 8ft for the East wall
leaving 13 ft.
Back to tan ratio.
tan 39.8 = 13/ adjacent
(Adjacent is distance from West wall)
Adjacent = 13/tan 39.8
Adjacent = 15.6ft
At 15.6 ft from the West wall you can use a 21 ft stepladder
so this is where to fix the light.
Hope this helps.
:-)