Write each expression in the form $2^k$ or $3^k$ for a suitable constant $k.$

$($a$)\frac{3^{6x}}{3^{4x}}$

$($b$)\frac{2^{(3x1)}}{2*2^{-3x}}$

$($c$)\frac{4^{-x}}{8^{-\frac{x}{3}}}$

$($a$)\frac{3^{6x}}{3^{4x}}=$