Answer:
58°
Step-by-step explanation: Nick was able to view the museum painting through the mirror due to reflection. The painting was reflected after hitting the plane mirror. According to the law of reflection, the angle of incidence is equal to the angle of reflection. The incident angle is the angle which the incident ray( the ray which strikes or hits the plane mirror) makes with the normal(line perpendicular to the tangent).
Thus, the angle which the painting makes with the mirror is equal to the angle at which the painting is being viewed = 58°.