Note that the side adjacent to angle XYZ is given (6 cm) and that the hypo is also given (15 cm). Thus, cos XYZ = adj / hyp = 6 cm / 15 cm = 2/5 = 0.4.
XYZ is then arc cos 0.4. Use the arccos function on your calculator to determine XYZ and give XYZ in both degrees and radians.