If the cube shown is 7 centimeters on all sides what is the length of the diagonal x.

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

7√3

x = 25

step-by-step explanation:

a^2+b^2=c^2

7^2+24^2=c^2

49+576=625

sqrt625=25

x=25