How do you find a missing value in a rectangle?

For a side, one side is equal to the other side. If you don't know the other side, even if you know one side and the perimeter/area, you can calculate it algebraically. For example, if you want to find the perimeter of a rectange with one side who's length is 7. Because the other side's length is 7, you can add that to get 14. Then use the perimeter formula P=2L+2W to calculate it. Suppose the perimeter is 30. You can plug that into the formula as well as put in 14 for 2L or just put in 7 for L. Then subract 14 from both sides of the formula to get 16=2W. Then just divide by 2 to get W=8, which would be equal to the missing sides of the rectangle.