(a)
Given, $P(A) = P(B) = x$
and $P(A \cap B) = P\left( A^{'} \cap B^{'} \right) = \frac{1}{3}$
$\therefore\ P\left( A^{'} \cap B^{'} \right) = 1 - P(A \cup B)$
$\Rightarrow \ P(A \cup B) = 1 - \frac{1}{3} = \frac{2}{3}$ …(i)
Also, $P(A \cup B) = P(A) + P(B) - P(A) \cap B)$
$\Rightarrow \ \frac{2}{3} = 2x - \frac{1}{3}$ [from Eq (i)]
$\Rightarrow \ x = \frac{1}{2}$