What is the answer to 21-9f=-20(f-6)

Proceed as in example 3 in section 6.1 to rewrite the given expression using a single power series whose general term involves xk. ∞ n(n − 1)cnxn − 2 n = 2 − 4 ∞ ncnxn n = 1 + ∞ cnxn n = 0

Ijust need these 2 questions answered (the second pic is just confirmation i'm not confident in that answer)

18 16 11 45 33 11 33 14 18 11 what is the mode for this data set