Answer
a) $|\theta_1|=|\theta_2|$
b) $r=2l\sin(\theta)$
Work Step by Step
a) $\theta_1=\arctan(|\frac{m_1g}{F_{12}}|)$
$\theta_2=\arctan(|\frac{m_2g}{F_{21}}|)$
$m_1=m_2$ and $F_{12}=-F_{21}$, so $|\theta_1|=|\theta_2|$
b) $\frac{r}{2}=l\sin(\theta)$
$r=2l\sin(\theta)$
