As it stands now, A and D are on the same horizontal line, and B and C are on the same horizontal line. However, we want to move only point B.So, if we keep BC parallel to AD, then we want to have BC the same length as Ad, which is 6. Right now, the length of BC is 5-1= 4. So, to increase the length to 6, staying on the line y=5, we move B to (-1, 5).