Arectangle's width is 2 meters shorter than its length ,/. it's area is 168 square meters . which equation can be used to find the length of te rectangle?

Let x be the length of the rectangle, then its width is x-2.
The area of a rectangle is L x  W OR x.(x-2). Expand it = x² - 2x = AREA.

Or x² - x = 168
x²-2x-168 = 0. This is a quadratic equation to solve for x:

x' = [-b+√(b²-4ac)]/2a and x" = [-b-√(b²-4ac)]/2a
you will find x' =14 and x" = -12 (extraneous value not to be considered), so 
x=14