Answer
The length of an edge is $5.16\dot{A}$.
Work Step by Step
The $SrO$ bond is formed when $Sr$ shares 2 electrons with $O$, thus the compound consists of $Sr^{2+}$ cations and $O^{2-}$ anions.
The length of the $SrO$ bond is the sum of the radii of the two ions. According to the figure the radii of $Sr^{2+}$ is 1.32 and the radii of $O^{2-}$ is 1.26. The length of the $SrO$ bond is therefore:
$1.32+1.26=2.58$.
The side of the cube in the figure consists of two $SrO$ bonds in a linear arrangement, so the length of the side is twice the length of one bond.
$2\times2.58=5.16$
