(4-2)   ①   Distance travelled is just velocity x time:   so the distance is just D = vt.

② The  general equation we derived in Question 4-1 was:         y = Asin(2π(x-D)/λ)

If you follow  the movement of a particular peak or trough as a function of time, in the diagram, you can see that the movement of the wave just shifts it in the same way as curve (d) was shifted in that question – but now the size of the shift increases with time. So all we need to do is to substitute in our expression for the time-dependent displacement:  the equation becomes:

           y = Asin(2π(x-vt)/λ)