Complex sentence: The displaced image of the source (slit) is at an angle 2 "θ" from the source direction since the angle is the sum of the angle "θ" of incidence of the beam and its angle of reflection.
Split into simpler sentences: The displaced image of the source (slit) is at an angle 2 "θ" from the source direction. This is because the angle by which the image is displaced is the sum of the angle "θ" of incidence of the beam and its angle of reflection.