$f '\left( x \right)=\frac{ \mathrm{d} }{ \mathrm{d}x} \left( {\left( {x}^{3}-6x \right)}^{7} \right)$
$f '\left( x \right)=\frac{ \mathrm{d} }{ \mathrm{d}g} \left( {g}^{7} \right) \times \frac{ \mathrm{d} }{ \mathrm{d}x} \left( {x}^{3}-6x \right)$
$f '\left( x \right)=7{g}^{6} \times \frac{ \mathrm{d} }{ \mathrm{d}x} \left( {x}^{3}-6x \right)$
$f '\left( x \right)=7{g}^{6} \times \left( 3{x}^{2}-6 \right)$
$f '\left( x \right)=7{\left( {x}^{3}-6x \right)}^{6} \times \left( 3{x}^{2}-6 \right)$
$f '\left( x \right)=\left( 21{x}^{2}-42 \right) \times {\left( {x}^{3}-6x \right)}^{6}$