2x + 1 - 1 = 13 - 1
2x = 12
2x/2 = 12/2
x = 6
on a bien 2 * 6 + 1 = 12 + 1 = 13
puis
-3x + 20 = 14
-3x + 20 - 20 = 14 - 20
-3x = -6
-3x/(-3) = -6/(-3)
x = 2
on a bien 20 - 3*2 = 20 - 6 = 14
et la dernière
2x + 1/7 - 1/7 = 1 - 1/7
2x = 6/7
x = 6/7 * 1/2
x = 6/14
x = 3/7
