5(1 - xy)(1 + xy)
expanding
$\displaystyle 5(1 - xy)(1 + xy)$
$\displaystyle (5-5xy)(1 + xy)$
$\displaystyle 5*1+5*xy-5xy*1-(5xy)(xy)$
$\displaystyle 5+5xy-5xy-5x^2y^2$
simplifying
$\displaystyle 5-5x^2y^2$
$\displaystyle 5-5(xy)^2$
$\displaystyle 5(1-(xy)^2)$
But if you know the formula mentioned in the other post, you can save yourself some of this trouble.
I am showing all the steps here (just in case) since the problem said "expand and simplify"