I don't understand the palindrome word a(b^n)a.

Based on the pumping lemma definition:
The pumping states that: |word| >=n
The length of this word is (1 + n + 1) which is greater than n. This holds.

The pumping states that: |xy| <= n

a(b^n)a can be represented as xyz, but I don't understand why y = b^n (in my textbook)

It means that x = a, y = b^n, z = a
but the length of xy is (1 + n) which is greater than n. This
doesn't hold that |xy| <= n...

(Sorry about this not being a Java question, this is the "designing part" of Java)
Thank you.