(4-5)  ①  Looking at the y = sin(x) curve, you can see that a trough follows a peak after π radians. Therefore you need to put this shift into the equation for the wave:

           y = Asin(2π(x-vt)/λ - π)

②  You can see from the symmetry of the y = sin(x) curve that sin(x- π)  =  -sin x, so:

                          sin(2π(x-vt)/λ - π)    =    -sin(2π(x-vt)/λ)

Alternatively use the identity sin(a-b)  =  sin a cos b – cos a sin b.

Since cos π = -1 and sin π  = 0,  we get: 

sin(2π(x-vt)/λ - π)   =  [sin(2π(x-vt)/λ) x -1]  -  [cos(2π(x-vt)/λ)  x 0]

                                        =  -sin(2π(x-vt)/λ)

③  Whichever way you arrive at it, this leads you to the superposed wave being defined by:

y1 + y2  =   Asin(2π(x-vt)/λ)  -  Asin(2π(x-vt)/λ)   =  0

That is: the waves completely cancel out: