x" + 7x + 6
= x" + x + 6x + 6
= x( x + 1 ) + 6( x + 1 )
= ( x + 1 )( x + 6 )
或者
= x" + 6x + x + 6
= x( x + 6 ) + ( x + 6 )
= ( x + 1 )( x + 6 )