Hello dear aspirant

Given that: Thickness = 0.002 m & Velocity = 5750 m/s

We know that: Velocity = Frequency × Length

5750 = f × 0.0002

f = 5750 / 0.002

f = 2.875 × 10^6 Hz

Therefore, the fundamental frequency of the sound wave in the crystal is 2.875 × 10^6 Hertz.

Hope this helps!!

All the best for your future endeavours