Question:

If x > 0 and y > 0, which of the following expressions is equivalent to $5981_w73_h39.png$?

A $5120_w72_h21.png$
Explaination

Since x > 0 and y > 0, x can be rewritten as $2373_w53_h21.png$ and y can be rewritten as $1281_w52_h22.png$. It follows, then, that $5981_w73_h39.png$ can be rewritten as $4703_w129_h51.png$
Because the numerator is a difference of two squares, it can be factored as $3514_w182_h46.png$. Finally, dividing the common factors of $(\sqrt x - \sqrt y )$ in the numerator and denominator yields $4106_w92_h21.png$.