where am i wrong in my calculation?why is \$V_{bc}=220∠120°-120°=220∠0°\$,not \$V_{bc}=220∠-120°-(120°)=220∠-240°\$
if Vab=220∠120°,then what isVbc,and Vca ?




For a positive phase sequence Vbc will have a zero angle and Vca will have an angle of -240 degrees.

Where you appear to be going wrong is trying to fit your thinking to a scenario when Vab is at zero degrees. Yes, the reference phase angle is usually defined along the horizontal right-hand axis but, it can be redefined for any angle providing that the sequence direction is respected. That new reference is defined in the question as a phase of 120 degrees.