1. Draw a sketch.

2. Let l denote the length of the rectangle and w the width. Then the area of the rectangle is:

3. Plug in l instead of x and calculate the y-value which is the width of the rectangle:

4. You now got the area as a function in l:

5. Differentiate a wrt l and solve the equation a'(l) = 0 for l. I've got

6. Now calculate the maxium area.