Vector Algebra: JEE Main 2018 Mathematics question

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