Question

Helppppppppppppppppppp
i dont know if its the answer i clicked or not

Answer 1

Regrouping terms in the sum as

x + (-5x + 5x) + (-9x + 9x) + … + (-21x + 21x) - 25x

you can see that all the middle terms cancel and you're left with

x - 25x = -24x