Hmm...this really feels like a trick question because we weren't given any variables (there aren't any numbers to plug in).
Since this question isn't solvable (from a mathematical perspective), the answer...is actually already in the equation! Let's break it down, shall we? :)
In D = -200t + 9000, we know that D = "remaining debt" and "t" = months. The "-200" represents Harry paying back the bank: this payback is what is decreasing his debt (which is why it's negative).
The question is asking us for "size of Harry's loan". I believe that the answer is 9000 (right in front of us!)
I hope this helps!