Answer
$\int_{-2}^{6} f(x) d x.$
Work Step by Step
We have
$$
\int_{0}^{8} f(x) d x+\int_{-2}^{0} f(x) d x+\int_{8}^{6} f(x) d x\\=
\int_{0}^{6} f(x) d x +\int_{-2}^{0} f(x) d x\\
=\int_{-2}^{6} f(x) d x.
$$
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