Question 998900
So common a question type that a lesson was made for it:


<a href="http://www.algebra.com/my/FIND-RECTANGLE-DIMENSIONS-USING-GIVEN-PERIMETER-AND-AREA.lesson?content_action=show_dev">http://www.algebra.com/my/FIND-RECTANGLE-DIMENSIONS-USING-GIVEN-PERIMETER-AND-AREA.lesson?content_action=show_dev</a>