Pls answer.

Let a be the edge of a cube

Volume of cube = a³

There are 12 edges in a cube, so total length of the edges of a cube = 12 a

Surface Area = 6a²

According to the condition given in the question,

a³ + 3 x 12 a = 2 x 6a²

a² + 36 = 12 a

⇒ a² - 12a +  36 

⇒ (a-6)(a-6)

 a = 6

The diagonal of a cube of side a is a3

This can be found by applying the Pythagorean Theorem twice (first to find the diagonal of a face of the cube, a2, and then to find the diagonal through the center, a3). 

Length of the diagonal of a cube =63