Answer
$P(\frac{5}{13},\frac{-12}{13})$
Work Step by Step
We have the following equation for the unit circle:
$x^{2}+y^{2}=1$, so we plug in $P(\frac{5}{13},y)$,
$(\frac{5}{13})^{2}+y^{2}=1$
$ y^{2}=1 - \frac{25}{169}= \frac{144}{169}$
$y = \frac{12}{13}$ or $y = - \frac{12}{13}$ but the y-coordinate is negative so $y = - \frac{12}{13}$
