∵n-2m=5
∴[(m²+n²)-(n-m)²+2n(m-n)]/2n
=(m²+n²-n²+2mn-m²+2nm-2n²]/2n
=(4mn-2n²)/2n
=2n(2m-n)/2n
=2m-n
=-(n-2m)
=-5