Compute the following limits:

$($a$)\underset{x+}{lim}\frac{1+a^x+(2a{)}^x}{1-a^x-(3a{)}^x}$ where $($i$)a=0\mathrm{.}4$

$($ii$)a=0\mathrm{.}8,$

$($b$)\underset{x+}{lim}(1-\frac{1}{2x}{)}^{3x}.$