691. Point $M$ has coordinates $(x, y)$. Find the coordinates of point $M^{\prime}$, symmetric to point $M$ with respect to the x-axis. The same question for the point symmetric to $M$ with respect to the y-axis, and also for the point symmetric to $M$ with respect to the origin.
$\triangleright$ Points $M$ and $M^{\prime}$ lie on the same vertical line, so their abscissas are the same. They are at the same distance from the x-axis but on opposite sides of it, so their ordinates are equal in magnitude but opposite in sign. Answer: $(x,-y)$. (Note that this answer holds true for $y=0$ and $y<0$ as well.)

The point symmetric to $M$ with respect to the y-axis has coordinates $(-x, y)$.

The point symmetric to $M$ with respect to the origin has coordinates $(-x,-y)$. (It can be noted that this point is obtained by successive reflections with respect to the x-axis and the y-axis, so both signs change.) $\triangleleft$