destructive interference

Xamfy19

Here is the problem:

The waves froma radio station can reach a home reciever by two paths. One is a straightline path from transmitter to home, a distance of 19900 m. THe second path is by reflection from the ionoshpere.
Assume: this reflection takes place at a point midway between reciever and transmitter. No phase changes on reflection. If the wavelength brodcast by the radio station is 464 m, find the minimum height h of the ionoshperic layer that produces destructive interference between the direct and the reflected beams.

I cannot figure out how to solve this problem. Where do I start? Could someone please help me?

Xamfy19

I got the answer. thank you for trying. the answer is 1.5237 km