The easiest way to understand the answer is to consider two straight lines.
y = 3x + 6
and 2y = 6x + 12
When you graph these two lines, one will be on top of the other. That will give you an infinite number of solutions.
The answer does not depend on the two equations being straight lines. All that is required is that one graph sits on the other. So you could have a pair of quadratics
y = x^2 + 3x + 10
2y = 2x^2 + 6x + 20
would also give you an infinite number of solutions.
