SOLUTION
We want to factor
[tex]y^2-400[/tex]This is difference of two squares, so we have
[tex]y^2-20^2[/tex]factoring, we have
[tex]\begin{gathered} y(y)-20(20) \\ paring\text{ items outside the bracket we have } \\ (y-20) \\ paring\text{ items in the bracket, we have } \\ (y+20) \\ =(y-20)(y+20) \end{gathered}[/tex]Hence the answer is
[tex](y-20)(y+20)[/tex]