20 how much metal is needed to smelt a cubical metal box with outer side 12 inches long if the thickness of its walls should be exactly 3 inches?

The volume of the metal box if the box was completely solid (V1), is:
 
 V1=(12 inches)³
 V1=1728 inches³
 
 As there are 3 inches of metal on both sides, the widht if the metal box was not completely solid, is:
 
 W=12 inches-(3 inchesx2)
 W=6 inches
 
 Then, the volumen of the no solid metal box is:
 
 V2=(6 inches)³
 V2= 216 inches³
 
 Therefore, the volume of metal needed to smelt the cubical metal box, is:
 
 V3=V1-V2
 V3=1728 inches³-216 inches³
 V3=1512 inches³

Idk i am in 5th grade lol