How do i solve 4/7 =b/7

Which property is illustrated by the following statement? if hxa rgi, then rgi hxa.

The area of a rectangle is 10 cm^2. one side of the rectangle is x cm. express the perimeter of the rectangle in terms of x and show that this rectangle cannot have a perimeter of 12 cm. the perimeter of the rectangle in terms of x is __ cm

Having a hard time understanding this question (see attachment).