$$\left. \begin{array} { l } { \frac { - x } { 3 } = 7 \quad 2 x - 5 = - 7 } \\ { 2 x - 5 = 5 \quad 2 k + 1 = 9 } \end{array} \right.$$
$x=\frac{15}{43}\approx 0.348837209$
$$-x=21\times 2x-15$$
$$-x=42x-15$$
$$-x-42x=-15$$
$$-43x=-15$$
$$x=\frac{-15}{-43}$$
$$x=\frac{15}{43}$$
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