Formula of hertz
Hertz is cycles per second. If you have the wave length (peak to peak or trough to trough) then you just measure the time between peaks in seconds (t). Hertz (H).
H = 1/t
That’s it. So if you have .5 second wave length in time, then you have 2 cycles per second or 2 Hertz.
f = 1/T
Not quite sure how to answer this
Hope this helps!