Question 310564
surface area of cube = 54 

let the  side be x
x^2 is the area of one face of the cube.
there are 6 faces.
6x^2 = 54
x^2= 54/6
x^2=9
x=3 units

apply pythagoras theorem to find the diagonal.

3^2 +3^2 = diagonal^2

9+9= diagonal ^2
diagonal = sqrt18