a) \(\left(\dfrac{x}{3}-4\right).5=15\)

\(\Leftrightarrow\dfrac{x}{3}-4=\dfrac{15}{5}\)

\(\Leftrightarrow\dfrac{x}{3}-4=3\)

\(\Leftrightarrow\dfrac{x}{3}=3+4\)

\(\Leftrightarrow\dfrac{x}{3}=7\)

\(\Leftrightarrow x=7.3\)

\(\Leftrightarrow x=21\)

Vậy \(x=21\)

b) \(128-3\left(x+4\right)=23\)

\(\Leftrightarrow3\left(x+4\right)=128-23\)

\(\Leftrightarrow3\left(x+4\right)=105\)

\(\Leftrightarrow x+4=\dfrac{105}{3}\)

\(\Leftrightarrow x+4=35\)

\(\Leftrightarrow x=35-4\)

\(\Leftrightarrow x=31\)

Vậy \(x=31\)

c) \(\left(3x-2^4\right).7^3=2.7^4\)

\(\Leftrightarrow3x-2^4=\dfrac{2.7^4}{7^3}\)

\(\Leftrightarrow3x-2^4=2.7\)

\(\Leftrightarrow3x-2^4=14\)

\(\Leftrightarrow3x-16=14\)

\(\Leftrightarrow3x=14+16\)

\(\Leftrightarrow3x=30\)

\(\Leftrightarrow x=\dfrac{30}{3}\)

\(\Leftrightarrow x=10\)

Vậy \(x=10\)

d) \(5x+x=39-3^{11}:3^9\)

\(\Leftrightarrow6x=39-3^{11-9}\)

\(\Leftrightarrow6x=39-3^2\)

\(\Leftrightarrow6x=39-9\)

\(\Leftrightarrow6x=30\)

\(\Leftrightarrow x=\dfrac{30}{6}\)

\(\Leftrightarrow x=5\)

Vậy \(x=5\)