Electronic watches keep accurate time using crystal oscillators. Inside the watch, there is a tiny block of quartz which vibrates. Two opposite faces of the block move alternately toward each other and away from each other. This is a caused by a standing wave in the block. The two opposite faces are at antinodes, and the plane halfway between these two faces is at a node. If the two faces are 5.85 mm apart and the speed of sound in quartz is 3.72 km/s, find the frequency of the vibration. kHz (+2 kHz)