The frequency of echo will be _____ Hz if the train blowing a...

The hill will be a secondary source.

When the train moves towards the hill then,

${f}_{1} =$ frequency of the train w.r.t. the hill is ${f}_{1} = \left(\dfrac{v}{v - {v}_{s}}\right)f = \left(\dfrac{330}{320}\right) \times 320 = 330Hz$

When the train driver hears an echo, then velocity of observer is ${v}_{0} = 36 \times \dfrac{5}{18} = 10m{s}^{- 1}$.

Now, ${f}_{2} =$ Frequency of the sound reflected by hill w.r.t. the car (echo)

${f}_{2} = \left(\dfrac{v + {v}_{0}}{v}\right){f}_{1} = \dfrac{\left(330 + 10\right)}{330} \times 330 = 340Hz$