Question 134416
What is the formula for the area of a rectangle?  Area = length * width.

Let length = l. 
Given: width = length - 4, Area = 320
{{{A = l*w}}}
{{{A = l * (l-4)}}}
{{{320 = l^2 -4l}}}
{{{0 = l^2 -4l - 320}}}
{{{0 = (l + 16)(l - 20)}}}
l = -16 , l = 20.

Since the length cannot be negative, the only solution that 'makes sense' is l = 20.

Check your answer. If l = 20, then width = 20-4 = 16. Is the area 320?