Answer
See explanations.
Work Step by Step
Letting $u=x-c$, we have $du=dx$ and $x=a\to u=a-c, x=b\to u=b-c$.
Start from the left-hand side
$LHS=\int_a^b f(x)dx=\int_{a-c}^{b-c}f(u+c) du=RHS$
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