Source and observer are moving in opposite direction, therefore, apparent frequency \(v^{\prime}=v \times \frac{(v+u)}{(v-u)}\)
\(\frac{9}{8} v=v \times \frac{340+u}{340-u}\)
\(\Rightarrow 9 \times 340-9 u=8 \times 340+8 u\)
\(\Rightarrow 17 u=340 \times 1 \Rightarrow u=\frac{340}{17}=20 m / sec\)