## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$-\dfrac{3}{5}$
Since the trigonometric function $\sin$ is an odd function, we can write $f(-\theta)=-f(\theta) \implies \sin{-a}=-\sin a$. We have: $f(x)=\sin x$ and $f(a)=\dfrac{3}{5}$ Therefore, $f(-a) =\sin (-a) =-\sin a=-\dfrac{3}{5}$