Question

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 $\overrightarrow{a}$and $\overrightarrow{u}.\overrightarrow{b}$= 24, then $\left|\overrightarrow{u}\right|$^2 is equal to-