a) \(12+x+\left(-5\right)=-18-2x\)
\(\Rightarrow12+x-5=-18-2x\)
\(\Rightarrow x+7+18+2x=0\)
\(\Rightarrow3x=-25\)
\(\Rightarrow x=-\dfrac{25}{3}\)
b) \(\left(-14\right)-x+\left(-15\right)=-10+\left(4-2x\right)\)
\(\Rightarrow-14-x-15=-10+4-2x\)
\(\Rightarrow-x-29=-2x-6\)
\(\Rightarrow-x+2x=-6+29\)
\(\Rightarrow x=23\)
c) \(x-\left(-19\right)-\left(-11\right)=-\left(3x+40\right)\)
\(\Rightarrow x+19+11=-3x-40\)
\(\Rightarrow x+30=-3x-40\)
\(\Rightarrow x+3x=-40-30\)
\(\Rightarrow4x=-70\)
\(\Rightarrow x=-\dfrac{35}{2}\)