This is pretty straight forward.
First the area of a rectangle is given by the product of the length and the width and in this case the area is known.
Second the length is an unknown and since we are doing algebra lets call it x
Finally the width is 4 less than the width which is (x-4) units....x will be in meters and the width is 4 meters less then the length.
Putting it together we have x(x-4) = 117 m^2
then
x^2 - 4x = 117
one could use the solution for the quadratic or one could plug in a few numbers as test for the length
10 by 10 is 100 square meters
10 by 14 is 140 square meters
13 by 9 is 117 square meters....x is 13 and x-4 is 9
I used approximation once I had determined the relationship between the area of the rectangle and the perimeter of the rectangle