We have
[tex](3x-8)•\mleft(4x+7\mright)[/tex]First we will multiply 3x by 4x, then 3x by 7, then -8 by 4x and then -8 by 7
[tex](3x\cdot4x)+(3x\cdot7)+(-8\cdot4x)+(-8\cdot7)[/tex][tex]12x^2+21x-32x-56[/tex]then we sum similar terms and we obtain the answer
[tex]12x^2-11x-56[/tex]