Arc AC is 21°
The equation 3x-1.5 is an inscribed angle. According to the Inscribed Angle Conjecture, the measure of an angle inscribed in a circle is one-half the measure of the intercepted arc. Therefore, 2(3x-1.5) = 3x+9 which would make x=4. Substitute in x in 3x+9 to get the arc measure, which is 21°
