I think I remember this from geometry my sophmore year of High school. If it’s a parallelogram, you double the length, subtract the result from the perimeter, and divide the result by two. Here’s an example problem. I’ll explain each step cause I don’t know how much of this you know:

Imaginary parallelogram

Perimeter=20

length=6

Width=W

the first equation to find the width would look like this:

2(6) + 2W=20 simplify by multiplying out the parentetical expresion resulting in:

12+2W=20 now simplify by getting rid of the twelve. subtract it from both sides.

12+2W-12=20-12

final result of:

2W=8 now divide both sides by 2 to isolate W

2W/2=8/2 (can’t find a division symbol so I used the /)

Therefore,

W=4

Now to find the area:

Area=length X width (this works for any parallelogram, not just squares and rectangles)

So…

if Length=6 and Width=W=4,

6X4=24=area

Remember the definiton of a parallelogram: 4 sides, the opposite sides are parallel. Includes squares and rectangles, but is not limited to them.

Can look like this:

_

/_/

Ok, that’s a really bad illustration, but I’m on a keyboard, so I can’t draw one for you. It’s like a slanted rectangle.

Hope this helps. And sorry if my explanations were too elementary, didn’t know how much you already knew about math.

Hope this helps.

Surely this question has other details. If this is a rectangle you must know the ratio of the length to the width. With that information you can calculate the missing side dimension from factoring the known side and area. That involves simple algebra.