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