If the 2 lines are the same, which they are, they are the same lines. That means there are an infinite amount of solutions. If they had the same slope but different y-intercepts, there would be no solutions because the lines are parallel. Any other pair of equations would have 1 solution.
When you simplify the term on the left of the equation, it is exactly the same as the term on the right. Therefore, you can substitute virtually anything in for variable "x" and it would still be correct.
