Vector Algebra

Let a vector $\overrightarrow c$ be coplanar with the vectors $\overrightarrow a = - \widehat i + \widehat j + \widehat k$ and $\overrightarrow b = 2\widehat i + \widehat j - \widehat k$. If the vector $\overrightarrow c$ also satisfies the conditions $\overrightarrow c \,.\,\left[ {\left( {\overrightarrow a + \overrightarrow b } \right) \times \left( {\overrightarrow a \times \overrightarrow b } \right)} \right] = - 42$ and $\left( {\overrightarrow c \times \left( {\overrightarrow a - \overrightarrow b } \right)} \right)\,.\,\widehat k = 3$, then the value of $|\overrightarrow c {|^2}$ is equal to :