Bonjour !
A = (2x + 4 ) + ( x +3 )
= 2x + 4 + x + 3
= (2+1)x + 4+3
= 3x + 7
B = (4x - 4 ) + ( x - 11 )
= 4x - 4 + x - 11
= (4+1)x -4-11
= 5x - 15
c = (3a +9 ) - ( a + 2 )
= 3a + 9 - a - 2
= (3-1)a + 9-2
= 2a + 7
d = (5a - 11 ) - ( a + 12 )
= 5a - 11 - a - 12
= (5-1)a -11-12
= 4a - 23
e = ( x + 3 ) + ( x + 7 ) + ( x + 12)
= x + 3 + x + 7 + x + 12
= (1+1+1)x + 3+7+12
= 3x + 22
f = (3x +9 ) - ( x - 5 ) - ( x - 8 )
= 3x + 9 - x + 5 - x + 8 ( - fois - = +)
= (3-1-1)x + 9+5+8
= x + 22
g = ( 4x - 5 ) - ( x - 2 ) - ( x + 4 )
= 4x - 5 - x + 2 - x - 4
= (4-1-1)x + -5+2-4
= 2x - 7
h = ( 3x + 2y ) - ( x + y ) - ( x - y )
= 3x + 2y - x - y - x + y
= (3-1-1)x + (2-1+1)y
= x + 2y
N'hésite pas si tu as des questions !