[tex](5x-4y)^2[/tex]
[tex](5x-4y)(5x-4y)[/tex]
Multiply each term in one bracket in turn by the terms in the other bracket
[tex](5x)(5x) = 25x^2[/tex]
[tex](5x)(-4y)=-20xy[/tex]
[tex](-4y)(5x)=-20xy[/tex]
[tex](-4y)(-4y)=16y^2[/tex]
Put them together we have
[tex]25x^2-20xy-20xy-16y^2[/tex]
[tex]25x^2-40xy-16y^2[/tex]
