Question

This is some sort of pythagoras thing

Answer 1

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