a,
$72 - 3x = 5x + 8$
$\Leftrightarrow 5x + 3x = 72 - 8$
$\Leftrightarrow 8x = 64$
$\Leftrightarrow x = 8$
b,
$3\left | x - 7 \right | = 21$
$\Leftrightarrow \left | x - 7 \right | = 7$
$\Leftrightarrow \left[ \begin{array}{l}x - 7 = 7\\x - 7 = -7\end{array} \right.$
$\Leftrightarrow \left[ \begin{array}{l}x = 14\\x = 0\end{array} \right.$
c,
$-7\left | x + 3 \right | = -49$
$\Leftrightarrow \left | x + 3 \right | = 7$
$\Leftrightarrow \left[ \begin{array}{l}x + 3 = 7\\x + 3 = -7\end{array} \right.$
$\Leftrightarrow \left[ \begin{array}{l}x = 4\\x = -10\end{array} \right.$
