The recurrence fn=fn−1+21f_n = f_{n-1} + 21fn=fn−1+21 with f1=18f_1 = 18f1=18 gives fn=18+(n−1)cdot21f_n = 18 + (n-1) \\cdot 21fn=18+(n−1)cdot21. What is f6f_6f6?
105105105
126126126
111111111
123123123