2. Let $x$ be an acute angle, and $\sin x=3 \cos x, \sin x \cdot \cos x$ is equal to

保留源文本的换行和格式，直接输出翻译结果。
2. $\frac{3}{10}$.

Construct a right triangle, with the opposite side of angle $x$ being $a$, then the other leg is $3a$, and the hypotenuse is $\sqrt{a^{2}+(3 a)^{2}}=\sqrt{10} a$. Therefore, $\sin x \cdot \cos x=\frac{a}{\sqrt{10 a}} \cdot \frac{3 a}{\sqrt{10 a}}=\frac{3}{10}$.