Don't use the pythagorean theorem on right triangles. Instead, you should memorize the most common special triangles, two of which are here. The triangle on the left can be recognized as a 306090 triangle because the hypotenuse is double one of the legs. The ratio of the sides is 1√32, which means x on this triangle should be 4√3.
The triangle on the right is a 454590 triangle, recognizable because the two legs are the same length. The ratio of the sides is 11√2, which means y on this triangle should be 4√2.
Since 4√3 is greater than 4√2, the answer is A.
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