8. (3 points) A fraction $\frac{b}{a}$, when its denominator is reduced by 2, i.e., $\frac{b}{a-2}$, it simplifies to $\frac{3}{4}$; if the original fraction's denominator is increased by 9, i.e., $\frac{b}{a+9}$, it simplifies to $\frac{5}{7}$, then $\frac{b}{a}=$ $\qquad$
8. (3 points) A fraction $\frac{b}{a}$, when its denominator is reduced by 2, becomes $\frac{b}{a-2}$, which simplifies to $\frac{3}{4}$; if the original fraction's denominator is increased by 9, it becomes $\frac{b}{a+9}$, which simplifies to $\frac{5}{7}$, then $\frac{b}{a}=-\frac{165}{222}$.
【Solution】Solution: The two new fractions, before simplification, have the same numerator, so we can first convert the two fractions to have the same numerator,
$$
\begin{array}{l}
\frac{3}{4}=\frac{15}{20}=\frac{30}{40}=\frac{45}{60}=\frac{60}{80}=\frac{165}{220} \\
\frac{5}{7}=\frac{15}{21}=\frac{30}{42}=\frac{45}{63}=\frac{60}{84}=\frac{165}{231}
\end{array}
$$

The denominators of the two new fractions should differ by 11.
So the two denominators are: 222 and 231;
The original fraction's denominator is: $220+2=222$ (or $231-9=222$), so the original fraction is $\frac{165}{222}$. Therefore, the answer is $\frac{165}{222}$.