13. Let $A_{n}, B_{n}$ be the sums of the first $n$ terms of the arithmetic sequences $\left\{a_{n}\right\},\left\{b_{n}\right\}$, respectively. If $\frac{A_{n}}{B_{n}}=\frac{5 n-3}{n+9}$, then $\frac{a_{6}}{b_{3}}=$
By the properties of an arithmetic sequence, we know that $A_{n}=k n(5 n-3), B_{n}=k n(n+9)$, so $\frac{a_{6}}{b_{3}}=\frac{5}{11} \cdot \frac{11 a_{6}}{5 b_{3}}=\frac{5}{11} \cdot \frac{A_{11}}{B_{5}}=\frac{5 \cdot 11-3}{5+9}=\frac{26}{7}$.