Find the value of x for the given triangle

Which is an equation for the nth terms of the sequence 12,15,18,21

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

Another type of subtraction equation is 16-b=7. explain how you would sole this equation then solve it.