Answer
$$-f(-x+7)+C$$
Work Step by Step
Given
$$\int f^{\prime}(-x+7) \mathrm{d} x$$
Let
$$ u=-x+7 \ \ \ \Rightarrow \ \ \ du = - dx$$
Then
\begin{aligned} \int f^{\prime}(-x+7) d x&=-\int f^{\prime}(u) d u\\
&=-f(u)+C\\
&=-f(-x+7)+C\end{aligned}