$$\left. \begin{array} { l } { 6 } \\ { \frac { 12 x + 8 } { 15 x } - \frac { 1 } { 12 x + 6 x } - \frac { 2 x } { 2 x + 21 } } \\ { 1 } \end{array} \right.$$
$6,\ \frac{903+1598x-36x^{2}}{90x\left(2x+21\right)},\ 1$
