A supersonic jet plane moves parallel to the ground at speed v= 0.75 mach (1 mach = speed of sound). The frequency of its engine sound is `v_0= 2kHz` and the height of the jat plane is h= 1.5 km. At some instant an observer on the ground hears a sound of frequency `v= 2v_0`. Find the instant prior to the instant of hearing when the sound wave received by the observer was emitted by the jet plane. Velocity of sound wave in the condition of observer `=340 m"/"s`.