Answer :
[tex]\\ \sf\longmapsto (x-y)^2-a(y-x)[/tex]
[tex]\\ \sf\longmapsto x^2-2xy+y^2-(ay+ax)[/tex]
[tex]\\ \sf\longmapsto x^2+y^2-2xy-ay-ax[/tex]
Done
[tex]\\ \sf\longmapsto (x-y)^2-a(y-x)[/tex]
[tex]\\ \sf\longmapsto x^2-2xy+y^2-(ay+ax)[/tex]
[tex]\\ \sf\longmapsto x^2+y^2-2xy-ay-ax[/tex]
Done