When x=0, the formula attained in part b) gives the answer that we obtained in part a) (the board without any extra weight on it). This does make sense that the rope would not hold any of the mass if it were exactly over the wall-board interface. In all our calculations, we used this point as the pivot point, so we didn't have to consider the force exerted by the wall on the board because its lever arm is zero. When the person is at x=0, all his weight is being supported by the wall exerting a force on the board at this interface.