Answer: 20 m
Since we know that a square has four equal sides, we can find out the sides by taking the square root of the given area, which is 200 m^2.
Now we know that the sides of the square are 10 radical 2. However, the question is asking for the length of the diagonal. We can find out the diagonal of the square by slicing up the square into two triangles by drawing a diagonal from one corner to the other. We know that a square has 4 right angles. And we also know that the triangles are going to be a 45-45-90 degree triangle. Since we already know the side which is 10 radical 2, all we have to do is multiply radical 2 to it which is equal to 10 x 2= 20m