Question 654977
Note: I'm going to use the variable 'x' in the place of 'a'. So I'll be working on {{{(x^2 - 21x + 108) / (x-12)}}}


Perform polynomial long division to get


<img src="http://i150.photobucket.com/albums/s91/jim_thompson5910/Algebra%20dot%20com/9-22-201212-42-36PM_zps6caff67b.png">


So {{{(x^2 - 21x + 108) / (x-12) = x-9}}}


which means  {{{(a^2 - 21a + 108) / (a-12) = a-9}}}