Vector Algebra: JEE Main 2020 Mathematics question

Let $\overrightarrow a$, $\overrightarrow b$ and $\overrightarrow c$ be three unit vectors such that ${\left| {\overrightarrow a - \overrightarrow b } \right|^2}$ + ${\left| {\overrightarrow a - \overrightarrow c } \right|^2}$ = 8. Then ${\left| {\overrightarrow a + 2\overrightarrow b } \right|^2}$ + ${\left| {\overrightarrow a + 2\overrightarrow c } \right|^2}$ is equal to ______.