$$ 0.7 = P(A \cup B ) = P(A) + P(B) - P(A \cap B)) $$
$$ 0.9 = P(A \cup B^c) = P(A) + 1 - P(B) - P(A \cap B^c) = \\ P(A) + 1 - P(B) - (P(A) - P(A \cap B) = 1 - P(B) + P( A \cup B) $$
Addition der beiden Gleichungen ergibt $$ 1.6 = 1 + P(A) $$ also $$ P(A) = 0.6 $$