The surface area of a cube is calculated using the following formula
[tex]S=6a^2[/tex]
If the surface area is given then the side length can be found and vice versa.
We have thus
[tex]302.46=6a^2 \\ 302.46/6=a^2 \\ a=\sqrt{302.46/6}\approx5.8\mathrm{in}[/tex]
Hope this helps.