check the picture below.
recall that in a square, all sides are equal, so if we make b = a, then we get that for one side, also recall that the area of a rectangle is length*width, so in this case,
[tex]\bf \stackrel{\textit{area of the square}}{\stackrel{length}{5\sqrt{2}}\cdot \stackrel{width}{5\sqrt{2}}}\implies 25(\sqrt{2})^2\implies 25\cdot 2\implies 50[/tex]
