A counter-example is an concrete example (or case) which disagrees with a given statement. It takes a single counter-example to disprove any given statement.
There are only two lines that have a y-intercept of zero (the condition of the statement), namely
y=2x and
y=-2x.
Out of these, one has a positive slope (2) and the other has a negative slope (-2).
Therefore the only line that is a counter example is the one with a positive slope, or
y=2x
