Let $\overrightarrow{u}$ be a vector coplanar with the vectors $\overrightarrow{a} = 2 \hat{i} + 3 \hat{j} - \hat{k} $ and $\overrightarrow{b} = \hat{j} + \hat{k}$. If $\overrightarrow{u}$ is perpendicular to and $\overrightarrow{a}$ and $\overrightarrow{u} \overrightarrow{b} = 24$, then $| \overrightarrow{u}|^2$ is equal to :
