Collect

Collect[expr, x] collects together terms involving the same powers of objects matching x.

Examples

Collect terms in x:

Collect[a*x + b*x + c*x^2, x]
(* c x^2 + (a + b) x *)

Collect with multiple variables:

Collect[Expand[(x + y + 1)^3], {x, y}]
(* 1 + 3 y + 3 y^2 + y^3 + (3 + 6 y + 3 y^2) x + (3 + 3 y) x^2 + x^3 *)

Please visit the official Wolfram Language Reference for more details.