Mark wants to get a fence around his square backyard. The backyard is 256 square meters?
Well, if it's a square background, then the length of two sides must be the same. So we can square root 256 to get the length of one side. You can plug that into a calculator, but it's easy to memorize that it is 16.
I don't really know what you want to know...but here are some possible answers:
Length of one side: 16 meters
The total length of fence needed: 64 meters
Correct me if I'm wrong. (Getting a lot of questions wrong these days...)