12.  Waves of frequency 150 Hz are produced in a string.  Find the
(a)  wavelength, (b) amplitude and (c) velocity of the waves in SI units.

1) The wavelength is the distance between adjacent identical parts of a wave, parallel to the direction of propagation. 
So wavelength = 25cm

2) Amplitude—maximum displacement from the equilibrium position of an object oscillating around such equilibrium position, so amplitude = 6cm

3)The wave velocity is the speed at which the disturbance moves.
velocity = wavelength x frequency 
             = 25 x 150
             = 3750cm/sec

Regards