Since the $3x$ is in parentheses, the $4$ in the exponent applies to both the $3$ and the $x$.

$81x^{4}x^{3}$

$81x^{(4 + 3)}$

$81x^{7}$

$(2^{(4-3)})·(3^{(1-2)})$

$2^1(3^{-1})$

$\frac{2}{3}$

See (a) and (b).