Simple way to see it is to multiply the value opposite of the axis you're flipping it over by -1. For example the point (-5,7) would become (5,7) when reflected over the y-axis, because the point is moving from the left to the right. This means the triangle would be (1,3) (5,3) and (5,7) when reflected over the y-axis. For reflection over the x-axis it would be (-1,-3) (-5,-3) and (-5,-7) because each point is moving down.
