The trick here is to not use sin and cos at all.
The -13/15 is the x coordinate of a line from the origin to the unit circle (the line with angle theta). That happens to be the cos(theta).
The y coordinate is the sin(theta).
You can draw this as a triangle where x^2 + y^2 = 1.
So y = √(1 - (13/15)^2) ≈ 0.49889. But your exact answer should be:
[tex]sin(\theta) = \sqrt{1-(\frac{13}{15})^2} = \frac{2\sqrt{14}}{15}[/tex]
