Hello Charlotte!
The period of a wave can be solved by using the equation:
t = 1/f t being time and f being frequency.
So, t = 1/6.5 Hz = 0.154 Seconds
Finding the frequency also uses the same formula only its reversed:
f = 1/t
So.. f = 1/0.154 Seconds = 6.5 Hz
I hope that helps!