Answer: the lenght of the sides of the square is 40 m
Explanation:
I will answer this in English, hope you can follow it or translate it. 
This can be translated to: 
"The area of a square is equal to the product of two of the sides. 
If the area is 1600m^2, which are the dimensions of the square?" 
First, the length of the 4 sides of a square is the same, let's write this as L. 
Then the area of the square is L*L = L^2. 
Then we have L^2 = 1600m^2 
now we can apply the square root to both sides and get: 
√(L^2) = L = √(1600m^2) = 40m 
The length of all the sides of this square is 40m