bonjour
f (x) = ( x + 3 )² - ( 2 x + 3 )² ( x + 3 )
f (x) = x² + 6 x + 9 - [ ( 4 x² + 12 x +9 ) ( x + 3 ) ]
f (x ) = x² +6 x + 9 - ( 4 x ³ + 12 x² + 12 x² + 36 x + 9 x + 27 )
f (x) = x² + 6 x + 9 - 4 x ³ - 24 x² - 36 x - 9 x - 27
f (x) = - 4 x ³ - 23 x² - 39 x - 18
f (x) = ( x + 3 ) ( x + 3 - 2 x - 3 )
f (x) = ( x + 3 ) ( - x )