Let the length be 
and the width be 
. Now, write the length in terms of the width. "The length is 15 cm. less than twice the width" in the form of an equation is
Let's plug that into the equation for the perimeter now. The perimeter of a rectangle is
, so the equation for the perimeter of this rectangle is
Plugging in the length in terms of width and solving for the width, we get
To find the length, we just have to plug in the width into the equation we wrote for the length in terms of the width:
Let's plug that into the equation for the perimeter now. The perimeter of a rectangle is
Plugging in the length in terms of width and solving for the width, we get
To find the length, we just have to plug in the width into the equation we wrote for the length in terms of the width: