If $\frac{m}{n} = \frac{4}{3}$ and $\frac{r}{t} = \frac{9}{14}$, find the value of $\frac{3mr - nt}{4nt - 7mr}$. The original answer is in \frac{a}{b} format, please give the value of a + b.

Let's think step by step and output the final answer within \boxed{}.
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