Multiply and simplify:

$\displaystyle \frac{{{x}^{{3}}+{y}^{{3}}}}{{{x}^{{2}}-{y}^{{2}}}}\cdot\frac{{{y}-{x}}}{{-{y}-{x}}}$

Enter the numerator and denominator separately in the boxes below. If the denominator is 1, enter the number 1. Do not leave either box blank. Answer: