f(x+2) = f(x) + f(x+1) f(11) = 91 Let f(12) = af(13) = 91 + af(14) = 91 + 2af(15) = 182 + 3a.This is also equal to 617.182 + 3a = 617 => a = 145f(10) = f(12) - f(11) = 145 - 91 = 54.