The longest diagonal of a 6 cm cube is: 10.39 cm
6.928 (rounded)
It is sqrt(3) times length of an edge. How to figure. The diagonal across one face is sqrt(2) * edge [make a right triangle: the hypotenuse will be that diagonal]. Now make another right triangle,...
The principal diagonal.
it has 6 faces, 12 edges, and 8 vertexes