How do I write an index in a summation?

I need to expand this summation, but the coefficients a_n are taken as constant, instead of a_1, a_2, ....a_n

I cannot define a_x, because they are unknown, and I need the symbolic answer, not a numerical one.

Also, how do I get the factors of x^1 first? (it starts win x^20)

PD: Can I write latex in this forum?