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exercice 3

a) 7x = 13  x=13/7 

b) x-3=12

x=12+3=15

c) 3x+10=28

3x=28-10

3x=18

x=18/3

x=6

exercice 4

a)4x+7=2x+13

4x-2x=13-7

2x=6

x=6/2

x=3

b) -3x-8 = -7x-4

-3x+7x=-4+8

4x=4

x=4/4

x=1

3) a) 7x = 13

         x= 13/7

b) x-3 = 12

    x = 12-3

    x= 9

c) -x/3 = 5

    -x = 5x3

   -x = 15

    x = -15

d) 3x + 10 = 28

     3x = 28-10

      3x = 18

      x = 18/3

      x =  6

4) a) 4x + 7 = 2x + 13

        4x = 2x + 13 - 7

        4x = 2x + 6

        4x - 2x = 6

        2x = 6

        x = 6/2

        x = 3

b) -3x - 8 = -7x - 4

    -3x = -7x - 4 + 8

    -3x = -7x + 4

   -3x + 7x = 4

    4x = 4

     x = 4/4

     x = 1