We follow the ‘new Cartesian sign convention’.
The pole of the mirror is taken to be at origin of the rectangular coordinate system.
Accordingly, we shall use the following convention regarding the sign of various quantities.
(a) All distances are measured from pole of mirror.
(b) The distances measured in the same direction as the incident light are taken as positive and those measured in the direction opposite to the direction of the incident light are taken as negative.
(c) Height measured upwards and perpendicular to x – axis are positive whereas for downwards and perpendicular to x – axis are negative.