It's impossible to tell exactly what you've done, but here's what it looks like. Using the corner at bottom left as centre you've drawn a circle with radius eq

It's impossible to tell exactly what you've done, but here's what it looks like. Using the corner at bottom left as centre you've drawn a circle with radius equal to the long side, then you've done the same with the corner at bottom right. That gives you the point above, but closest to, the rectangle. Then you've joined that point to the corner at bottom left and marked the point where it crosses the top line. There is no reason why the length of the line crossing the rectangle should be equal to the right section of the divided top line. The top line of the rectangle can be moved up and down, and the two line lengths will vary in opposite directions. At some point they'll be equal, but in general they won't, whereas the problem requires that all four sides of the rhombus have equal length. If that's not clear I can write a longer description with diagrams, but I hope that explains why what you've got isn't a solution, even though it looks good.