Since all sides of a square are equal, finding the missing side is very straightforward. Here's what you can do:
If you know the length of one side (s):

The missing side of the square will also have the same length (s) as all sides in a square are congruent.
If you know the area (A) of the square:

You can find the side length (s) using the formula for the area of a square: Area (A) = side (s)².

To find the missing side length, perform the following steps:

Take the square root of the area (√A).

This will give you the value of the side length (s) of the square.
Here's an example:
Imagine you have a square with an area of 16 square meters (A = 16 m²).

Find the side length (s):

s = √(A) = √(16 m²) = 4 meters
Therefore, each side of the square would be 4 meters long.
Remember:

You only need one piece of information, either the length of a side or the area of the square, to find the missing side length.

All squares have four equal sides.