The formula for the area of a rectangle is length times width, or A=l·w
The Area A=256
The width is w
The lenght is given as 6 feet less than 4 times its width, or in math language this is l=4w-6
Let's go back to the formula
A=l·w ⇒ 256=(4w-6)·w
simplify ⇒ 256=4w²-6w
put everything on the same side and set the equation equal to zero
⇒ 4w²-6w-256=0
Solve for w using quadratic formula
You are going to get two answers after using the quadratic formula: one positive and one negative. Just remember it doesn't make sense to get a negative answer if we are talking about width, so use the positive answer only
w≈8.7851 units