2x" - x - 21
= 2x" - 7x + 6x - 21
= x( 2x - 7 ) + 3( 2x - 7 )
= ( x + 3 )( 2x - 7 )
或者
= 2x" + 6x - 7x - 21
= 2x( x + 3 ) - 7( x + 3 )
= ( x + 3 )( 2x - 7 )