RESOUDRE LES SYSTEMES SUIVANTS AVEC LA METHODE DE TON CHOIX (3x=y+14 (3y-2x+21=0 2x+5y=3y-2y-59 7x+9y=3y-5y+78

3x = y+14
3y-2x+21 = 0

3x-y = 14
-2x+3y = -21

par substitution
-y = -3x+14
on remplace y ds -2x+3y = -21
-2x-3(-3x+14) = -21
-2x+9x-42 = -21
7x = -21+42
x = 3

3x-y = 14
9-y = 14
-y = 14-9
y = - 5

3x - y = 14
3y - 2x= - 21

3x - y = 14
- y = 14 - 3x
y = 3x - 14

3y - 2x= - 21
3(3x - 14 ) - 2x= - 21
9x  - 42  - 2x = -21
7x = -21 + 42
7x = 21
x = 21/7
x = 3

y = 3x - 14
y = 3(3) - 14
y = 9 - 14
y = - 5