Question

A plane mirror approaches a stationary personwith acceleration a. The acceleration of his image , as seen by the person,will be

Answer 1

If the acceleration of the person is a, the acceleration of the image will be -a (negative sign is to indicate that it is in the opposite direction)
So the acceleration of the image as seen by the person is the relative acceleration of the image and the person.

relative acceleration = a - (-a) = 2a

Answer 2

Person is stationary.  Mirror is moving with an acceleration of a towards him.  With respect to the mirror, the image distance of the object is reducing at an acceleration a.  So distance of image wrt mirror is reducing with the same acceleration. 

Acceleration of the image wrt stationary person =  acceleration of the image wrt to mirror + acceleration of the mirror with respect to the person.
         = a + a = 2 a  towards the person.

In the above expression, the accelerations are vectors and they are added as they both are in the same direction.