Answer
$5x^2+17x-12$
Work Step by Step
Using $(a+b)(c+d)=ac+ad+bc+bd$ or the product of $2$ binomials, the given expression, $
(5x-3)(x+4)
$, is equivalent to
\begin{array}{l}\require{cancel}
5x(x)+5x(4)-3(x)-3(4)
\\\\=
5x^2+20x-3x-12
\\\\=
5x^2+17x-12
.\end{array}