find like terms and group them
3n and 5n are same and 4n^2 and 4n^2 are same
but 3mn and 5n are not and 3n and 2n^3 are not
group like terms
(-2m^2)+(-2mn+1mn)+(8)+(-10m)+(10n-5n)+(2n^2)
add like terms
(-2m^2)+(-mn)+(8)+(-10m)+(5n)+(2n^2)
-2m^2+2n^2-10m+5n-mn+8 is simplest form