Alright, This one is fairly simple its just some Subtracting :)
Graham has to pay $22,000
Well if his debt went from $22,000 to $15,200
He had to of paid some off
So let me put down the Equation, It's very simple ( The word problem could be hard though).
$22,000 - $15,200 this Equals $6,800
He Paid $6,800 in the Following year.
Therefore Your Answer Would be $6,800 :)