Answer
The length is $=\sqrt {54}$ units.
Work Step by Step
Length$=\sqrt {(2)^{2}+(0)^{2}+(-5)^{2}+(5)^{2}}$
$=\sqrt {4+0+25+25}$
$=\sqrt {54}$, since a length must be greater than zero (cannot be negative)
Therefore the length is $=\sqrt {54}$ units.
