11. Given that $A B C D$ is a parallelogram, if $\overrightarrow{A B}=\vec{a}, \overrightarrow{A C}=\vec{b}, E$ is the midpoint of $C D$, then $\overrightarrow{E B}=$ $\qquad$
11. $\frac{3}{2} \vec{a}-\vec{b}$.