Find the slope ($m$) and $y$-intercept ($b$) of the boundary line defined by $3x+6y=12$.

$m=-\frac{1}{2}$, $b=2$

$m=-3$, $b=12$

$m=2$, $b=-\frac{1}{2}$

$m=-\frac{1}{6}$, $b=12$