5m²-[5m²-(2m²-mn)-7mn-5]
= 5m²-5m²+（2m²-mn）+7mn+5 去括号
= 2m²-mn+7mn+5 合并同类项,去括号
= 2m²+6mn+5
= 2(m²+3mn)+5
己知
m²+3mn=5
代入上式
2 × 5 +5
= 15