Need help

AC  is a diameter

arc ABC  =180

angle ABC measures (1/2)  of this arc  = 90

minor arc BC  =  2 * minor arc AB

So  let minor arc AB  = x

So   minor arc BC  = 2x

So minor  arc AB + minor arc BC  =180

            x              +  2x   =  180

3x  = 180

x  = 180/3   = 60

If minor arc AB  = 60   then  angle ACB  is  (1/2)  of this   =  30