The frequency of this signal is?

If you are given a graph of the signal. Calculate the period, T, which length of time for one cycle.

The frequency is f = 1/T (Hz)

For example. T = 20 ms, f = 1/(20 ms) = 50 Hz.

If you are given an equation, in the form of:

v(t) = Vm cos (ωt + ϴ)

The frequency is ω (rad/s)

For example v(t) = 120 V cos ((314 rad / s) t + 0)

The frequency is 314 rad/s = 50 Hz.