The length of a rectangle is twice the width. the area is 256 squared. find the length and the width.

The solution is quite simple. 
First, draw a rectangle. Label two opposite sides as "w", representing the width. Label the other two sides as "2w", since the length is twice the width.
Next, write an equation. In this case, the equation would be 256=2w*w.
Simplify the equation to 2w^2=256. Divide both sides by two to get w^2=128. The width would be the square root of 128. The length would be twice that, so it would be the square root of 256. That would be 16, so the length is 16.