According to the law of reflection, the incident ray (that falls on the mirror), reflected ray (that is reflected off the mirror) and the normal to the mirror, all lie on the same plane. More importantly, the angle of incidence and angle of reflection are equal, or
angle of incidence = angle of reflection.
Angle of incidence and angle of reflection are the angles that incident ray and reflected ray, makes with the normal (to the mirror), respectively.
In this case, the angle of incidence is given as 35 degrees, thus the angle of reflection is also 35 degrees. As stated earlier, the incident and reflected rays make this angle with the normal. Thus, the angle between the incident and the reflected rays is the sum of angle of incidence and angle of reflection, that is, 70 degrees (= 35 + 35 degrees).
Hope this helps.