Write the Word or Phrase That Best Completes Each Statement $$\cos 3 x = \cos ^ { 3 } x - 3 \sin ^ { 2 } x \cos x$$

Write the word or phrase that best completes each statement or answers the question.
- $$\cos 3 x = \cos ^ { 3 } x - 3 \sin ^ { 2 } x \cos x$$