Area of a square: side²
200=side²
side=√200=14.14
Pythagoras therorem:
hypotenuse²=leg₁² +leg₂²
In this case:
hypotenuse=diagonal
leg₁=leg₂=side
Therefore:
hypotenuse²=(14,14 m)²+(14,14 m)²
hypotenuse²=200 m²+200 m²
hypotenuse²=400 m²
hypotenuse=√(400 m²)=20 m
The diagonal would be 20 m.