Given D = -200t + 9000, if D models Harry´s ramaining debt and "t" the months after he took the loan, it implies that 9000 is the total loan. Then
D = -200t + 9000 ⇒ D - 9000 = -200t ⇒ (D - 9000)/(-1) = 200t
⇒ 9000 - D = 200t . The difference 9000 - D, is the sum that he pays each month. So, first month he pays 200 dollars, the second 400 dollars, the third 600.... and so on.
