( x - 1 )( x - 2 ) = 6
x" - x - 2x + 2 = 6
x" - 3x - 4 = 0
x" + x - 4x - 4 = 0
x( x + 1 ) - 4( x + 1 ) = 0
( x + 1 )( x - 4 ) = 0
解方程得
x1 = -1,
x2 = 4