(a+y)(a-y)

The expression \((a+y)(a-y)\) is a difference of squares. It can be simplified using the formula for the difference of squares, which states that:

\[ (x+y)(x-y) = x^2 - y^2 \]

Applying this to your expression, we can identify \(x = a\) and \(y = y\):

\[ (a+y)(a-y) = a^2 - y^2 \]

So, the simplified form of the expression \((a+y)(a-y)\) is:

\[ a^2 - y^2 \]