Two sound sources 3 m apart have the same frequency of 870 Hz. The sources emit energy in the form of spherical waves. An observer initially hears a minimum sound, since the sources are out of phase, at a distance of 11 m along the perpendicular bisector of the line joining the two sources. What distance x must the observer move along a line parallel to the line joining the two sources before hearing another sound of minimum intensity? The speed of sound is 355 m/s.