Area of a rectangle is equal to length times width so the equation:
[tex]A = lw[/tex]
To find w, you would solve for it in this equation by dividing by l on both sides like so:
[tex] \frac{A}{l}= \frac{lw}{l} [/tex]
Which leaves you with:
[tex]w = \frac{A}{l} [/tex]
So to find width you divide the Area by the length:
[tex]w = \frac{A}{l}[/tex]