Well, since it is a cube, we know the length of all the sides of 10 cm, so we can create a triangle using the sides of a square and use pythagorean theorem A²+B²=C² C being the hypotenuse, or the diagonal we are looking for, so 10²+10² = 200 Then we square root to find C, so √200 = 14.14213562
In the problem it says that x is the number of adult shirts which 15 and y is the number for a youth shirt. So it would be 15x 12y. Hope this helped! :D
