Answer
$-\dfrac{3}{5}$
Work Step by Step
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,
$f(-a) =\sin (-a) =-\sin a=-\dfrac{3}{5}$
