Answer:
using Pythagoras' theorem c²=a²+b²
the diagonal is the hypotenuse of one of the triangles formed
let x represent one side of the square
√48²=x²+x²
√48²=2x²
48=2x²
48/2=2x²/2
24=x²
√24=√x²
4.8989794855663561=x
~4.90
Area of the square=side x side
4.90x4.90
24.01units²