smc

First we want to put this is slope-intercept form, so we get $y=\frac{1}{3}x+\frac{11}{3}$. When we reflect a line across the x-axis, the line's x-intercept stays the same, while the y-intercept and the slope are additive inverses of the original line. Since $m+b$ is the sum of the slope and the y-intercept, we get $-\frac{1}{3}-\frac{11}{3}=\boxed{-4}$.