-Let a (cm) be the width of the small rectangles, then 2a is their length and the perimeter of the rectangle PQRS is : 2(2a + a + 2a) = 10a

The area of each small rectangle is : 1536 : 3 = 512 (cm^2)

We have : 2a.a=512⇔a^2=256⇔a=162a.a=512⇔a2=256⇔a=16

The answer is : 16 x 10 = 160 (cm)