The angle of incidence is always equal to the angle of reflection. To get the angle of incidence draw a normal (perpendicular) line to the surface of the mirror. The angle between the ray coming from the object and this normal line will be the angle of incidence. If it is a perfect mirror, this will be the angle of reflection as well.

The question probably refers to a situation where the object being seen on the mirror is large such that the different edges of the object have different reflecting angles; in other words one cannot see the object as a “point”. This is a case of generalization; you must view the object as made up of multiple points. The ray from each of these points will have different angle of incidences, and for each point the angle of incidence will be the angle of reflection.

So, in short, follow the ray from each point of the object, see where it strikes the mirror, draw a perpendicular to the mirror’s surface, get the angle of incidence and you will know the angle of reflection.

Hope this helps.
Thanks,