I really understand but its still a bit confusing because before the addition of OH-, what i have in the flask? NH3, NH4+ and OH-, NH3 Is 15.3 all the way, NH4+ and OH- are both x (that we can calculate).
after the addition that x should become smaller because the reaction goes to the left, so NH3 increase a tiny bit but we ignore that, but the concentration of NH4+ should decrease (x-y) and the OH- should also decrease so it would be like x+0.008-y, do you get my point?