(a)
$P\left( A^{'} \cap B^{'} \right) = 1 - P(A \cup B) = \frac{1}{3}$ [given]
$\Rightarrow \ P(A \cup B) = \frac{2}{3}$
$\therefore\ P(A \cup B) = P(A) + P(B) - P(A \cap B)$
$\Rightarrow \ \frac{2}{3} = x + x - \frac{1}{3}\ \Rightarrow \ x = \frac{1}{2}$