Picture 1 shows the reflection of ABC 180° clockwise about the origin. To draw A'B'C' we draw the line segments AO, BO, and CO and extend them such that O is the midpoint of line segments AA', BB', and CC' respectively.
Picture 2 shows the line y=-x (the yellow line.) This is the line containing all points of the form (x, -x), that is, this points contains (1, -1), (-2, 2) etc...
We see that point C' is already on this line, and we draw the reflections of points B and C with respect to this line, to find B'' and C'' with coordinates as shown in the figure.
like with sound - the further away you are, the lower the volume of the sound (as the sound volume is the indicator of the amplitude of a sound wave, or the brightness of a light is the indicator of the amplitude of a light wave).
the same for any other type of wave (like the seismic wave of an earthquake).
so, the larger the distance, normally the smaller the amplitude.