let A be a subset of a space (X,T) and let A' is the derive set (the set that contains the clusters points ) show that {y} is open in the subspace

I will try to show that A-{y} is closed by prove that

(y)={y} is in not in (X,T) and it is enough to prove that

Suppose that

but

so

here where I stuck can anyone help me ??

Thanks