[tex]{ \sf{ \: factorise \: \: \: { \underline{ \green{2 {x}^{5} - 7 {x}^{3} - 4x}}}}} \\ \\ { \dashrightarrow{ \blue{ \sf{taking \: x \: common}}}}[/tex]
[tex]{ \leadsto{ \sf{ \green{ \:x(2 {x}^{4} - 7 {x}^{2} - 4)}}}} \\ \\ { \dashrightarrow{ \sf{ \blue{let \: {x}^{2} be \: equal \: to \: a}}}} \\ \\ { \leadsto{ \sf{ \green{ \: x( 2{a}^{2} - 7a - 4)}}}}[/tex]
[tex]{ \leadsto{ \sf{ \green{x(2 {a}^{2} - (8a \: - a) - 4}}}} \\ \\ { \leadsto{ \sf{ \green{x(2 {a}^{2} - 8a + a - 4)}}}} \\ \\ { \leadsto{ \sf{ \green{x(2a(a - 4) + 1(a - 4))}}}}[/tex]
[tex]{ \leadsto{ \sf{ \green{x(2a + 1)(a - 4)}}}} \\ \\ { \sf{ \: now \: putting \: the \: value \: of \: a}}[/tex]
[tex]{ \leadsto{ \green{ \sf{x(2 {x}^{2} + 1)( {x}^{2} - 4)}}}} \\ \\ { \leadsto{ \green{ \sf{ x( {x}^{2} + 1)( {x}^{2} - {2}^{2} )}}}} \\ \\ { \leadsto{ \green{ \sf{x( {x}^{2} + 1)(x + 2)(x - 2)}}}}[/tex]
