The question is not perfectly clear, but I assume you're asking something like this:
in a rectangle, the area is given by [tex]A=wl[/tex]
where A is the area, w is the width and l is the length.
So, if we know that the width is 7 and we let x be the length of the rectangle, we have
[tex]A=7x[/tex]
If you need to solve this for x, divide both sides by 7:
[tex]x=\dfrac{A}{7}[/tex]
