Answer
$$
\frac{L-\pi r_2-2d}{\pi}, \ \ 3.00
.$$
Work Step by Step
To solve the given equation for $r_1$, we move out the term $\pi r_2+2d$ and then divide both sides by $\pi$ as follows
$$
L =\pi(r_1 + r_2) + 2d\Longrightarrow \pi r_1=L-\pi r_2-2d\\
\Longrightarrow r_1=\frac{L-\pi r_2-2d}{\pi}
.$$
Now, by substitution, we have
$$
r_1=\frac{L-\pi r_2-2d}{\pi}=\frac{37.68-5\pi -2(6.28)}{\pi}=3.00
.$$
