2. Let $x$ and $y$ both be positive numbers. Then the minimum value of $M=\frac{4 x}{x+3 y}+\frac{3 y}{x}$ is $\qquad$ .
2.3.

Notice that, $M=\frac{4 x}{x+3 y}+\frac{x+3 y}{x}-1 \geqslant 3$. When $x=3 y$, $M=3$.