Right triangles $△ABC$ and $△XYZ$ are similar with $\angle B=\angle Y=90^{\circ }$ and corresponding sides $AB\leftrightarrow XY$, $BC\leftrightarrow YZ$, $AC\leftrightarrow XZ$. If $AB=7$, $BC=24$, $AC=25$, and $XY=21$, what is the length of $YZ$?