#### Answer

$6x^{3}-x^{2}-13x-4$

#### Work Step by Step

We will expand the expression by multiplying each term of the first expression by the whole second expression. Then, we will collect similar terms to obtain:
$(3x+1)(2x^{2}-x-4)$
=$3x(2x^{2}-x-4)+1(2x^{2}-x-4)$
=$6x^{3}-3x^{2}-12x+2x^{2}-x-4$
=$6x^{3}-3x^{2}+2x^{2}-x-12x-4$
=$6x^{3}-x^{2}-13x-4$