smc

Rearranging, we find $3x+y=-2x+6y$, or $5x=5y⟹x=y$. Substituting, we can convert the second equation into $\frac{x+3x}{3x-x}=\frac{4x}{2x}=\boxed{\text{(D)}2}$. More step-by-step explanation: $\frac{3x+y}{x-3y}=-2$ $3x+y=-2\left(x-3y\right)$ $3x+y=-2x+6y$ $5x=5y$ $x=y$ $\frac{x+3y}{3x-y}=\frac{1+3\left(1\right)}{3\left(1\right)-1}=\frac{4}{2}=2$. We choose $\boxed{2}$.