A rectangle has length 72 cm and width 56 cm. A second rectangle has the same area as this one, but its width is 21 cm. What is the length of the second triangle?
The area of the first rectangle is 72 cm x 56 cm = <<72*56=4032>>4032 cm^2.
Let x be the length of the second rectangle.
The area of the second rectangle is x cm x 21 cm = 4032 cm^2.
Thus, x = 4032 cm^2 / 21 cm = 192 cm.
The length of the second rectangle is 192 cm. Answer: \boxed{192}.