If the width of a rectangle is 3in less than the length, and the perimeter of the rectangle is 14in, find the length and width of the rectangle.

You can write equations for the length (l) and width (w) as
.. l -w = 3
.. 2(l +w) = 14

Divide the second equation by 2 and add the first equation.
.. (2(l +w))/2 +(l -w) = (14)/2 +3
.. 2l = 10
.. l = 5
Then
.. w = l -3 = 2

The length and width are 5 inches and 2 inches, respectively.