Um when doing this, there is no defined b term yet--b is unknown still. So it would be y= 2x+b. Then you have to plug (1,2) in as x and y, and solve for b. Once you get the answer for b, you put it into your equation and have your answer. But saying you want y=2x-3 to pass through (1,2) is impossible unless it already passes through it since all of the terms are already definite and put into place so they can't be changed.
