Answer
Length = $x^{3}-2$ units
Work Step by Step
Factoring the expression for the area,
$5x^{4}-10x=5x\cdot x^{3}-5x\cdot 2$
$=5x\cdot(x^{3}-2)$
Since the area of a rectangle equals (length)$\times$(width),
and one factor of the area is 5x, the width,
it follows that the other factor must be the length.
Length = $x^{3}-2$ units.
