Answer
The equivalent trigonometric form of $-21 - 20i$ is $29cis223.60^\circ$ or $29(cos223.60^\circ + isin223.60^\circ)$.
Work Step by Step
For $-21 - 20i$, the absolute value is $\sqrt{(-21)^2 + (-20)^2} = 29$ and $\theta = arctan(\frac{-20}{-21}) = 223.60^\circ$, since $-21 - 20i$ is in the 3rd quadrant.
Therefore, the equivalent trigonometric form of $-21 - 20i$ is $29cis223.60^\circ$ or $29(cos223.60^\circ + isin223.60^\circ)$.
