The wording on the problem was not correct. It is supposed to have the same setup as problem 7.8 on the handout, without saying that $\widehat{AB}$ was $135^\circ$. 

The problem should say "Suppose $\omega$ is a circle with radius $6$ and center $O$. Let $A$, $B$ and $C$ be points on $\omega$ such that $\overline{OA} \parallel \overline{BC}$. What angle does $\widehat{AB}$ have to be so that $OACB$ is a parallelogram?"

The problem is now updated on the HW and will be given correct to all students.