The graph shows an angle $a$ in standard position with its terminal side intersecting the circle at $P\left(-\frac{21}{29}, \frac{20}{29}\right)$. The center of the circle is the origin and the radius of the circle is 1.

Find the value of $\tan a$.

Please directly answer the question and provide the final value, e.g., 1, 2.5, 300.
Question: The graph shows an angle $a$ in standard position with its terminal side intersecting the circle at $P\left(-\frac{21}{29}, \frac{20}{29}\right)$. The center of the circle is the origin and the radius of the circle is 1.

Find the value of $\tan a$.

Please first conduct reasoning, and then answer the question and provide the final value, e.g., 1, 2.5, 300, at the end.
Question: The graph shows an angle $a$ in standard position with its terminal side intersecting the circle at $P\left(-\frac{21}{29}, \frac{20}{29}\right)$. The center of the circle is the origin and the radius of the circle is 1.

Find the value of $\tan a$.
