The two signals arive with a phase difference of 65m (143-78) or 2.5 wave lengths. You can drop the integral portion so the two signals effectively arrive a half wave appart.
Can you answer the question from here?
Two sources emit radio waves that are coherent, in phase and have identical wavelengths of 26 m. Do the waves interfere constructively or destructively at an observation point that is 78 m from one source and 143 m away from the other? Explain.