To find frequency, we apply the definition of frequency: frequency is the number of times a wave repeats in a given amount of time. Usually, frequency is measured in repeated cycles per second which is commonly called the Hertz (Hz). According to this problem, the wave is being forced to repeat twice each second, therefore the frequency is 2.0 Hz.
Period is defined as the reciprocal of the frequency. Therefore the period for this problem is T = 1/f = 1/(2.0cycles per second) = 0.50 seconds. T = 0.50s
Wave speed can be determined in several ways. One way is to determine the speed which must be required for the wave front to move across the distance between repeated cycles. In this case d = 0.14m in 0.50s. s = d/t = 0.14m/0.50s = 0.28m/s
Or one could get the wave speed by solving one of the following formulae
s = d/T
s = df
in either case you should still get s = 0.28m/s