Bonsoir,
A= (x+3) ( 2x+1 -10x -12)
= (x+3) ( -8x -11)
B= (x-4) ( 2x+9 +21x +9)
= (x-4) ( 23x +18)
C= (5x+2) ( 3x-1 -9x -8)
= (5x+2) ( -6x-9)