CoefficientList

CoefficientList[poly, var] gives a list of coefficients of powers of var in poly, starting with power 0.

Examples

Get all coefficients:

CoefficientList[3x^3 + 2x^2 + x + 5, x]
(* {5, 1, 2, 3} *)

For a quadratic:

CoefficientList[x^2 - 3x + 2, x]
(* {2, -3, 1} *)

Please visit the official Wolfram Language Reference for more details.