functions – xp to level formula

So I have this function to calculate how much xp is needed to get to a level.
25 * level * (1+level)

but im having trouble making a xp to level function. I have tried a few things that people on Discord gave me.

(25 * xp) + ((25 * xp)^2)

((xp / 25) / (1-xp))

just to name a few. but none work.

this is made with code, but i thought id ask here since its more math related than code related.