x - {34 - [26 + ( -66 - x)]} = 27 - { 43 + [ 25 - (20 - x )]}
⇔x - {34 - [26 - 66 -x ]} = 27 - { 43 + [25 - 20 - x]}
⇔x - {34 - 26 + 66 + x} = 27 - {43 + 25 - 20 - x}
⇔x - {74 + x} = 27 - {47 - x}
⇔x - 74 + x = 27 - 47 + x
⇔(x-x) -74 = x -20
⇔-74 =x -20
⇒ x = -74 +20 =-54
\(x-\left\{34-\left[-40-x\right]\right\}=27-\left\{43+5+x\right\}\\ \Leftrightarrow x-\left\{74+x\right\}=-21-x\\ \Leftrightarrow-21-x=-74\\ \Leftrightarrow-x=-53\\ \Leftrightarrow x=53\)