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

$81x^4x^3$

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

$81x^7$

$(2^{(4 − 3)}) · (3^{(1 − 2)})$

$2^1(3^{−1})$

$\frac{2}{3}$

See (a) and (b).