Hold a mirror along the x-axis and left-right stays put while up-down flips, so y switches sign to give (x, -y); mirror along the y-axis and it's x that switches instead.
Module goal. Turn reflection, point symmetry, and translation into simple add-subtract and sign-flip rules on coordinates, laying the groundwork for transforming function graphs later
eastmath.com · 20.2 Symmetry and Shifting in Coordinates · 20.2.1 Reflection Across an Axis