11. \( \frac{5}{21} + b = 2 \frac{1}{3} \)

\( b = 2 \frac{1}{3} - \frac{5}{21} = \frac{7}{3} - \frac{5}{21} = \frac{49}{21} - \frac{5}{21} = \frac{44}{21} \)