Bonjour je n'ai pas du tout compris ce DM de math niveau 4ème...pouvez vous m'aider ?

Exercice A1:

a) 9x + 7 = 28

9x + 7 - 7 = 28 - 7

9x = 21

9x/3 = 21/3

3x = 7

b) 3x + 5 = 5x + 8

3x + 5 - 3x = 5x + 8 - 3x

5 = 2x+8

5 - 8 = 2x + 8 - 8

-3 = 2x

-3/2 = 2x/2

x = -1,5

c) – 3 – 8x = – 3x – 6

-8x + 3x = -6 + 3

-5x = -3

-5x/-5 = -3/-5

x = 0,6

d) 2(x + 3) = 4( x – 1)

2x + 2*3 = 4x - 4*1

2x + 6 = 4x - 4

2x + 6 - 2x = 4x - 4 - 2x

6 = 2x - 4

6 + 4 = 2x - 4 + 4

10 = 2x

10/2 = 2x/2

x = 5