1) $$7m(m^3-8m^2+9)=7m^4-56m^3+63m$$
2) $$(x-2)(2x+3)=2x^2+3x-4x-6=2x^2-x-6$$
3) $$(3m-4n)(5m+8n)=15m^2+24mn-20mn-32n^2=15m^2+4mn-32n^2$$
4) $$(y+3)(y^2+y-6)=y^3+y^2-6y+3y^2+3y-18=y^3+4y^2-3y-18$$
Ответ: 1) $$7m^4-56m^3+63m$$, 2) $$2x^2-x-6$$, 3) $$15m^2+4mn-32n^2$$, 4) $$y^3+4y^2-3y-18$$