Answer
$0$
Work Step by Step
Since the grouping symbol is preceded by a negative sign, then the sign of each term inside the grouping symbol alters once the grouping symbol is removed. Hence, the given expression, $
5x^4+3x-(5x^4+3x)
$, is equivalent to
\begin{array}{l}\require{cancel}
5x^4+3x-5x^4-3x
.\end{array}
By combining like terms, then this simplifies to
\begin{array}{l}\require{cancel}
(5x^4-5x^4)+(3x-3x)
\\\\=
0
.\end{array}
