The angle of reflection is always equal to the the angle of incidence, which is the angle that the incoming ray, called incident ray makes with the normal.
(Please note that the angle of incidence and angle of reflection are always defined in terms of angle the ray makes with the normal. There is nothing like "angle of reflection with respect to mirror)
The angle of incidence can be calculated from the angle incident rays makes the reflection surface as:
Angle of incidence = 90 - Angle made with reflecting surface
The angle with the reflecting surface, that is mirror, is given as 35 degrees.
Therefore:
Angle of incidence = 90 - 35 = 55 degrees
And
Angle of reflection = angle of incidence = 55 degrees