Answer
$T=1.75\approx 1.8s$, $f=0.57Hz$
Work Step by Step
The frequency of oscillation is simply the amount of revolutions per second. This can be found using the known values of $12$ cycles and $21$ seconds to get a frequency of $$f=\frac{12cycles}{21s}=.57Hz$$ The formula relating frequency and period is $$T=\frac{1}{f}$$ Substituting the known value of $f=.57Hz$ yields a period of $$T=\frac{1}{0.57Hz}=1.75\approx 1.8s$$
