∠acb = 8°
step-by-step explanation:
∠xba lies on the circle and is half the measure of the intercepted arc ax
hence
measure of arc ax = 2 × 42° = 84°
∠acb is outside the circle and is
∠acb = (arcab - arcax ), hence
∠acb = 0.5 (100 - 84) = 0.5 × 16° = 8°