I'm not sure, but I think the answer is every point is moved so that it is three times as far from the y-axis. A triangle is congruent to another triangle if it has the same shape and size as the triangle. As long as every point is moved the same distance, the triangle should still be congruent to the figure shown. If you multiply each point by three times as far as it is from the y-axis, the points won't be moved the same amount and then it isn't congruent.
