$\frac{ 3{x}^{2}-x-21x+7 }{ 3x-1 }$
Factor out $x$ from the expression$\frac{ x \times \left( 3x-1 \right)-21x+7 }{ 3x-1 }$
Factor out $-7$ from the expression$\frac{ x \times \left( 3x-1 \right)-7\left( 3x-1 \right) }{ 3x-1 }$
Factor out $3x-1$ from the expression$\frac{ \left( 3x-1 \right) \times \left( x-7 \right) }{ 3x-1 }$
Cancel out the common factor $3x-1$$x-7$