Instead of plugging in [tex]m=-2[/tex] and [tex]n=4[/tex] right away, notice that
[tex]m^2-2mn+n^2=(m-n)^2[/tex]
and now evaluating this expression at the two given values is easier, if only a little:
[tex]((-2)-4)^2=(-6)^2=(-1)^2\times6^2=1\times36=36[/tex]
