You seem to be missing a symbol, which I assume is intersection [tex]\cap[/tex].
From the definition of conditional probability,
[tex]\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[/tex]
