13. Given complex numbers $z_{1}, z_{2}$ and $\left|z_{1}\right|-1,\left|z_{2}\right|=2.5,\left|3 z_{1}-2 z_{2}\right|=7$. Find $\frac{z_{1}}{z_{2}}$.
13. Given $z_{1} \overline{z_{1}}=1, z_{2} \overline{z_{2}}=\frac{25}{4}$
$$
\begin{aligned}
& \left(3 z_{1}-2 z_{2}\right)\left(3 z_{1}-2 z_{2}\right)-49 \\
\Rightarrow & 9 z_{1} \overline{z_{1}}+4 z_{2} \overline{z_{2}}-6\left(z_{1} \overline{z_{2}}+z_{1} z_{2}\right)=49
\end{aligned}
$$

We get $z_{1} \overline{z_{2}}+\overline{z_{2}} z_{2}=\frac{5}{2}$, so $\frac{z_{1}}{z_{2}}, \frac{25}{4}$. $\frac{z_{2}}{z_{1}}=\frac{5}{2}$.
Let $t=\frac{z_{1}}{z_{2}}$, then $\frac{25}{4} t+\frac{1}{t}=\frac{5}{2}$
Solving for $t$ gives $t=\frac{1}{5}\left(1=\sqrt{3} 3_{1}\right)$.
Therefore, $\frac{z_{1}}{z_{2}}=-\frac{1}{5}(1-\sqrt{3} i)$.