Vector Algebra: JEE Main 2021 Mathematics question

Let $\overrightarrow a$, $\overrightarrow b$, $\overrightarrow c$ be three mutually perpendicular vectors of the same magnitude and equally inclined at an angle $\theta$, with the vector $\overrightarrow a$ + $\overrightarrow b$ + $\overrightarrow c$. Then 36cos²2$\theta$ is equal to ___________.