Đáp án:
$\begin{array}{l}
b)2x + 12 = 3\left( {x - 7} \right)\\
\Rightarrow 2x + 12 = 3.x - 3.7\\
\Rightarrow 3x - 2x = 12 + 21\\
\Rightarrow x = 33\\
Vậy\,x = 33\\
c)2{x^2} - 1 = 49\\
\Rightarrow 2{x^2} = 49 + 1\\
\Rightarrow 2{x^2} = 50\\
\Rightarrow {x^2} = 50:2\\
\Rightarrow {x^2} = 25\\
\Rightarrow x = 5\,hoặc\,x = - 5\\
Vậy\,x = 5\,hoặc\,x = - 5\\
d)\left| { - 9 - x} \right| - 5 = 12\\
\Rightarrow \left| { - 9 - x} \right| = 12 + 5\\
\Rightarrow \left| { - 9 - x} \right| = 17\\
\Rightarrow \left[ \begin{array}{l}
- 9 - x = 17\\
- 9 - x = - 17
\end{array} \right.\\
\Rightarrow \left[ \begin{array}{l}
x = - 9 - 17 = - 26\\
x = - 9 + 17 = 8
\end{array} \right.\\
Vậy\,x = - 26\,hoặc\,x = 8
\end{array}$
