We know that a light ray strikes a plane at an angle of 50 degrees from the normal. This means that the light ray is striking the mirror at an angle of 40 degrees from the mirror. This is so because the normal and the mirror are always at a right angle.
This "initial" light ray that strikes is called the incident ray.
The laws of reflection tell us that the angle (from the normal) made by the incident ray is always equal to the angle made by the reflected ray (from the same normal). Hence, the reflected ray will also have a 50 degree angle from the normal. This is, like mentioned, equivalent to 40 degrees from the mirror.