$$\frac { 12 } { 7 } r - \frac { 1 } { 21 } r = r - \frac { 4 } { 3 }$$
$r=-2$
$$\frac{5}{3}r=r-\frac{4}{3}$$
$$\frac{5}{3}r-r=-\frac{4}{3}$$
$$\frac{2}{3}r=-\frac{4}{3}$$
$$r=-\frac{4}{3}\times \frac{3}{2}$$
$$r=\frac{-4\times 3}{3\times 2}$$
$$r=\frac{-4}{2}$$
$$r=-2$$
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