Okay... on air, sound travels at 342 meters per second
Then the frequency of sound is 1/t (q stands for the time period)
Then we know that speed is equal to distance over time (s=d/t)
Then the final equation would be like this s=dt ---> s=dx (1/t) --->
342 meters/50 hz = 6.84 meters. The wavelength of 50 Hz of sound in air is 6.84 m.
This would be the answer if that sound in the problem is travelling in air. If it is travelling in water, or in solid, then its another story.