Answer
$-\frac{1}{4}$.
Work Step by Step
The sine function is odd.
So,
$f\left( -a \right)=-f\left( a \right)$
Substitute into the main expression:
$\begin{align}
& f\left( a \right)+2f\left( -a \right)=f\left( a \right)-2f\left( a \right) \\
& =-f\left( a \right) \\
& =-\frac{1}{4}
\end{align}$
Thus, the value of the given expression is, $-\frac{1}{4}$.
