Question

If P(A)= 0.3 and P(B|A)=0.9 what is P(A B)?

Answer 1

You seem to be missing a symbol, which I assume is intersection $\cap$.

From the definition of conditional probability,

$\mathbb P(B|A)=\dfrac{\mathbb P(B\cap A)}{\mathbb P(A)}\implies0.9=\dfrac{\mathbb P(B\cap A)}{0.3}\implies\mathbb P(B\cap A)=0.27$