Never mind, after experimenting a bit, I figured out the answer.
Essentially, I started by assuming \(F_c=F_{b+1}.\) Thus, \( F_c-F_b = F_b+F_{b-1}-F_{b} = F_{b-1}\)
Knowing that Fa, Fb, and Fc form an arithmetic sequence, we can use properties of Fibonacci numbers to write Fa, Fb, and Fc in terms of
Fb, and use the fact that a+b+c=2000 to find what one of them is. The rest is simple.
Fa=F665, a=b-2, c=b+1
I know I used LaTeX at the start, and it looks clear, but it was too hard to write quickly.