Speed of a helicopter determined by sonar

[raman911]

Homework Statement

A helicopter flies directly toward a vertical cliff. when the helicopter is 700 m from the cliff face it transmits a sonar signal. it recives the reflected signal 3.4 s later. if the signal travels at a wind speed 350m/s, what is the speed of the helicopter.

The Attempt at a Solution

250*3.4= 1190 m
please help me what i need to do next? i spend 1 hour on it i did n't get answer.

 

[mgb_phys]

How far did the signal travel in 3.4s?
If the helicopter was stationary it would have to have traveled 2*700=1400m
How much less did it travel?
The helicopter must have traveled that far in 3.4s

 

[raman911]

How far did the signal travel in 3.4s?
If the helicopter was stationary it would have to have traveled 2*700=1400m
How much less did it travel?
The helicopter must have traveled that far in 3.4s

what u mean by 2*700?

 

[mgb_phys]

The distance the sound would have to travel 700m to the cliff and 700m back to the helicopter = 1400m if the helicopter was stationary.
In fact it only goes 3.4*350 = 1190m so the helicopter must have moved how much closer?