Ok i understood your answer, but why did mine not work??

Your integration is incorrect. You cannot treat x like a constant by divide its differentiation, x is a variable, not a constant.
Morever, you do not have a formula to integrate square root of a²-x². You only have formula to integrate 1 over square root of a²-x².

Dn rlly get u, but its ok!! I will clarify with my teacher, thanks alot for ur help!