2a" - ab - 2b"= (1/2)( 4a" - 2ab - 4b" )= (1/4)(1/2)( 16a" - 8ab - 16b" )= (1/8)[ (4a)" - 2b(4a) + b" - b" - 16b" ]= (1/8)[ ( 4a - b )" - 17b" ]= (1/8)( 4a - b - √17b )( 4a - b + √17b )