$解:(1)原式=3\ \mathrm {\ \mathrm {m^2}}n+3mn-4\ \mathrm {\ \mathrm {m^2}}n+4mn-2mn=-\ \mathrm {\ \mathrm {m^2}}n+5mn$
$(2)由題意得當(dāng)m=-2,n=1時(shí)$
$\ \mathrm {\ \mathrm {m^2}}n+mn=2,\ \mathrm {\ \mathrm {m^2}}n-mn=6,mn=-2$
$∴3×2-4×6◆2×(-2)=12,即6-24◆(-4)=12$
$∴24◆(-4)=-6$
$∴◆代表的是“÷”$