5m^2-(5m^2-(2m^2-mn)-7mn-5)
=5m^2-5m^2+(2m^2-mn)+7mn+5
=(2m^2-mn)+7mn+5
=2m^2+6mn+5,
又因为m^2+3mn=5,
所以2m^2+6mn=10,
∴原式=2m^2+6mn+5=15.
5m^2-(5m^2-(2m^2-mn)-7mn-5)
=5m^2-5m^2+(2m^2-mn)+7mn+5
=(2m^2-mn)+7mn+5
=2m^2+6mn+5,
又因为m^2+3mn=5,
所以2m^2+6mn=10,
∴原式=2m^2+6mn+5=15.