Try this explanation:
1. if to simplify the equation into the form ax²+bx+c=0 then to calculate 'D' ⇒ x²-8x-20=0; D=16+20=36. The value of D is greater 0, it means the given equation has two solutions.
2. Wrong step and correct way are shown in the picture in the attachment.
3. Description of the solution: the points with x=1;3 are the intersection points of the function y=x²-4x+3 and Ox-axis.
PS. all the details are in the attachment.