Verified answer

[tex]1)~(x+3)(x-7)-4x(5-2x)=x^2-7x+3x-21-20x+8x^2=\\\\=9x^2-24x-21;\\\\2)~(y+2)(y-6)+(y+3)(y-4)=y^2-6y+2y-12+y^2-4y+3y-12=\\\\=2y^2-5y-24;\\\\3)~(z-3)(3z+1)-(2z+3)(4z-1)=\\\\=3z^2+z-9z^2-3-8z^2+2z-12z+3=-14z^2-9z;\\\\4)~(2a-3b)(7a+4b)-(3,5a+b)(4a-6b)=\\\\=14a^2+8ab-21ab-12b^2-14a^2+21ab-4ab+6b^2=4ab-6b^2;\\\\5)~(m^3-3n)(m^2+2n)-4m^3(m^2+7n)=\\\\=m^5+2m^3n-3m^2n-6n^2-4m^5-28m^3n=\\\\=-3m^5-26m^3n-3m^2n-6n^2.[/tex]