Answer
$r= \frac{16}{3}$
Work Step by Step
This is a linear function. Solve for $r$. $$
\begin{aligned}
-3(r-8)+7 & =15 \\
-3(r-8) & =15-7 \\
-3(r-8) & =8 \\
r-8 & =-\frac{8}{3} \\
r & =8 \cdot \frac{3}{3}-\frac{8}{3} \\
& =\frac{24-8}{3} \\
& =\frac{16}{3}.
\end{aligned}
$$ Define the following version of the function and plot it. We see that the zero is where it should be. $$
f(r) = 15+3(r-8)-7=3r-16.
$$
