Two sinusoidal waves, identical except for phase, travel in the same direction along a string, producing a

Question:

Two sinusoidal waves, identical except for phase, travel in the same direction along a string, producing a net wave y'(x, t) = (3.0 mm) sin (20x – 4.0t + 0.820 rad), with x in meters and t in seconds. What are?
(a) The wavelength λ of the two waves,
(b) The phase difference between them, and
(c) Their amplitude ym?