1. You have correctly shown that the measures of the arcs other than AB and CD are each 84°, so the total of AB and CD is 360° -2*84° = 192°. That amount is divided in the ratio 1:2, so the smallest portion (AB) is 192°/3 = 64°.
2. Arc BC has a measure that is double that of ∠BAC. That angle's measure can be determined from
.. cos(∠BAC) = BA/AC = 5/16
.. ∠BAC = arccos(5/16) ≈ 71.79°
.. arc BC = 2*71.79° ≈ 144°
3. Opposite angles in a cyclic quadrilateral are supplementary.
.. ∠D = 180° -108° = 72°
