Answer
The edge is $6 \ ft$ long.
Work Step by Step
The surface area of a cube can be computed as:
$S=6a^2$
where $a$ shows the edge of a cube.
$216=6a^2$
or, $a^2=36$
or, $a=\sqrt {36}=6 \ ft $
Thus, we can find that the edge is $6 \ ft$ long.
