In the given figure, PQRS is a rectangle and RTUS is a rhombus. The length of ST is 8 cm. 

If the area of rectangle PQRS is 56 cm², then what is the measure of RU?

It is known that parallelograms on the same base and between the same parallel lines are equal in area.

Now, parallelogram PQRS and RTUS lie on a common base SR, and between the same parallel lines RS and QU.

∴ Area of PQRS = Area of RTUS
⇒ 56 = $\frac{1}{2}$ × ST × RU

⇒ 56 =$\frac{1}{2}$× 8 × RU

⇒ 56 = 4 × RU

⇒ RU = 14 cm

Thus, the length of RU is 14 cm.