Suppose:
width=x in
length=2x-6
when 3 inches square is cut from the corner, the new dimensions will be:
width=(x-6) in
length=(2x-6-6)=(2x-12) in
height=3 in
thus the volume will be:
(x-6)(2x-12)3=1536
solving the above we get:
x=-10 or x=22
thus the original:
width=22 in
length=22*2-6=38 in