Answer
$-\frac{3}{5}$
Work Step by Step
We know that $\sin$, $\csc$, $\tan$ are odd functions, which means $f(-\theta)=-f(\theta).$
We also know that $\cos$, $\sec$, are even functions, which means $f(-\theta)=f(\theta).$
Since sine is an odd function, then
$f(-a) = \sin{(-a)}=-\sin{a}=-\frac{3}{5}$
