(a)
$\overrightarrow{\mathbf{c}} \bullet \left( \overrightarrow{\mathbf{b}} + \overrightarrow{\mathbf{c}} \right) \times \left( \overrightarrow{\mathbf{a}} + \overrightarrow{\mathbf{b}} + \overrightarrow{\mathbf{c}} \right)$
$= \overrightarrow{\mathbf{c}} \bullet \left( \overrightarrow{\mathbf{b}} \times \overrightarrow{\mathbf{a}} + \overrightarrow{\mathbf{b}} \times \overrightarrow{\mathbf{c}} + \overrightarrow{\mathbf{c}} \times \overrightarrow{\mathbf{a}} + \overrightarrow{\mathbf{c}} \times \overrightarrow{\mathbf{b}} \right)$
$= \overrightarrow{\mathbf{c}} \bullet \overrightarrow{\mathbf{b}} \times \overrightarrow{\mathbf{a}}$