Pleaase i will report if not correct!

The interior angle and extrior angles are supplemtary, meaning they add to 10

so if interior is 108 then

108+ext=180
ext=72

exterior is 72 degrees

2008      63.1
2009      63.1 - 1.7
2010      63.1 - 1.7*2
2011      63.1 - 1.7*3

x            63.1 - (x - 2008) *1.7

2018      63.1 - (2018 - 2008) * 1.7

63.1 - (10) * 1.7 = 63.1 - 17 = 46.1

Change from 2008 - 2018 = 63.1 - 46.1 =   17

% of change = change * 100 / initial value = 17*100/67.1 = - 25. 3%.

The negative sign indicates that the change was a decline, instead of an increase.

- 25.3%