If you know the perimeter and lenght of a rectangle and want to find its width do you divide the perimeter by the length?

No. If you draw a rectangle there are four sides, the length of two of those sides are equal to the length, and the other two are equal to the width. Therefore: the perimeter, which is equal to all the sides added up, is equal to two times the length plus two times the width: or 2L+2W= Perimeter. If you know L and you know the perimeter you can solve it algebraically. the width equals the perimeter minus two times the length, all that divided by two, or W= (P-2L)/2 (when you do the algebra). you might be thinking about the area in which case if you knew the area and the length and you want to find the width you would divide the area by the length.