$
\begin{aligned}
& \text { } \int_{-a}^a f(x) d x-\int_0^a f(-x) d x \\
& \because \int_{-a}^a f(x) d x=\int_0^a f(x) d x+\int_0^a f(-x) d x \\
& \Rightarrow \int_0^a f(x) d x
\end{aligned}
$
by using $(\mathrm{a}+0-\mathrm{x})$ property
$\Rightarrow \int_0^a \mathrm{f}(\mathrm{a}-\mathrm{x}) \mathrm{dx}$