OK, I see what you mean.
My bet is that the Zn diagram is given just as an example and is not directly related to the problem.
Haven't worked with Pourbaix diagrams in ages, so I can be missing something, but first reaction is not pH dependent - so it will produce just a horizontal line on the diagram.
Second reaction is not potential dependent, so it will produce a vertical line.
Thermodynamic data should be probably enough to calculate the slope for the reaction c, and it must go through the point of intersection for the a and b reactions. I admit I am guessing here.