6. Let $x \neq y$, and the sequences $x, a_{1}, a_{2}, a_{3}, y$ and $b_{1}, x, b_{2}, b_{3}, y, b_{4}$ are both arithmetic sequences, then $\frac{b_{4}-b_{3}}{a_{2}-a_{1}}=$ $\qquad$ .
Let the common differences of the two arithmetic sequences be $d_{1}, d_{2}$, then $d_{1}=\frac{y-x}{5-1}=\frac{y-x}{4}$, $d_{2}=\frac{y-x}{5-2}=\frac{y-x}{3} \Rightarrow \frac{b_{4}-b_{3}}{a_{2}-a_{1}}=\frac{2 d_{2}}{d_{1}}=\frac{8}{3}$.