Determine the intervals on which the function $h$ is increasing or decreasing, given its derivative $h^{\prime }\left(x\right)=(x+15)(x+21)(x-13)$.

Increasing on: $(-\(\infty\),-21)\(\cup\)(-13,15)$

Decreasing on: $(-21,-13)\(\cup\)(15,\(\infty\))$

Increasing on: $(-21,-13)\cup (15,\infty )$

Decreasing on: $(-\infty ,-21)\cup (-13,15)$

Increasing on: $(-\(\infty\),-21)\(\cup\)(-15,13)$$(-21,-15)\(\cup\)(13,\(\infty\))$

Decreasing on: $(-21,-15)\(\cup\)(13,\(\infty\))$$(-\(\infty\),-21)\(\cup\)(-15,13)$

Increasing on: $(-21,-15)\(\cup\)(13,\(\infty\))$

Decreasing on: $(-\(\infty\),-21)\(\cup\)(-15,13)$