\($x_n = x_{n - 1} + x_{n - 2}$ for all $n \ge 3$\)
x11 = 99 + x1
x9 + x10 = 99 + (x3 - x2)
(x7 + x8) + (x8 + x9) = 99 + (x5 - x4) - (x3 - x1)
(x5 + x6) + (x6 + x7) + (x6 + x7) + (x7 + x8) = 99 + x5 - x4 - x3 + x1
3x6 + 3x7 + x8 = 99 - x4 - x3 + x1
3x6 + 3(x5 + x6) + (x6 + x7) = 99 - (x6 - x5) - ( x5 - x4) + (x3 - x2)
7x6 + 3x5 + x7 = 99 -x6 +x4 + x3 - x2
8x6 + 3(x7 - x6) + x7 = 99 + x4 +( x5 - x4) - (x4 - x3)
5x6 + 4x7 = 99 + x5 - x4 + x3
5x6 + 4(x5 + x6) = 99 + x5 - (x6 - x5) + (x5 - x4)
10x6 + 4x5 = 99 + 3x5 -x4
10x6 + x5 = 99 - x4
10x6 + x5 + x4 = 99
10x6 + x6 = 99
11x6 = 99 divide both sides by 11
x6 = 9