Answer:
Side= 

Explanation:
Let one side of square be x, so we can write the area of square formula:

Now, we know area = 200, so we find x:

To get this answer to simplest radical form, we need to know the rule shown below:

Let' simplify our answer:

This is the simplest form since 
 can't be broken down further.
 can't be broken down further.