Question 322932
First some simplification,
{{{-m^2+2mn-n^2=-(m^2-2mn+n^2)}}}
{{{-m^2+2mn-n^2=-(m-n)^2}}}
.
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The difference of two squares.
{{{x^2+2mn-m^2-n^2=x^2-(m-n)^2}}}
{{{x^2+2mn-m^2-n^2=(x+m-n)(x-m+n)}}}