There's no cube shown but we can guess the diagonal is the long diagonal.
In a rectangular parallelpiped (aka a box) with dimensions a × b × c the long diagonal x satifies a 3D version of the Pythagorean Theorem,
x² = a² + b² + c²
In a cube a=b=c
x² = 3a²
x = a√3
Answer: 7√3
