Evaluate $(x+y)(x-y)$ when $x=13$ and $y = 5$.
I know that $(x+y)(x-y)$ is a special product that can be simplified using the formula $x^2 - y^2$. So I can substitute $x=13$ and $y=5$ into that formula and get $13^2 - 5^2$. Now I just need to calculate the squares and subtract them. I remember that $13^2 = 169$ and $5^2 = 25$. So $13^2 - 5^2 = 169 - 25 = 144$. 