My words may not have been as clear as I intended, but I think that the equations are correct. The equations define the distances between the points in two dimensions, not just the distance along the horizontal axis. (That's where Pythagoras comes in.) The first two equations define the distances between the first and second points (0.2) and the second and third points (2). The third equation ensures that all three lie on the same line. Also, D(x1) should be specified to be b+f.