step 1
find the volume of the square box
volume=area of the base*height
area of the base=10²-----> 100 units²
height=4 units
volume square box=100*4----> 400 units³
step 2
volume rectangular box=2*volume square box
volume rectangular box=2*400------> 800 units³
volume rectangular box=L*W*H
H=5 units
W=10 units
Volume=800 units³
800=L*10*5------> solve for L
L=800/(10*5)------> L=16 units
the answer is
the length of the rectangular box is 16 units
