It sure is. The 3d Pythagorean Theorem, with diagonal d of a rectangular prism with sides a,b,c is
d² = a² + b² + c²
Here that's
d² = 5² + 5² + 5² = 3(5²)
d = 5√3
I hate ruining a nice exact answer with a decimal approximation but they asked.
d ≈ 8.66
Answer: 8.7 cm