Решите системы уравнений 2x+y=-7 y+3x=11 x+y=3 7x-15y=-129 3x-y=-2 2x-y=9 2x-3y=-4 -3x-11y=-3

1) 2x + y = -7
+
3x - y = -2

5x = -9
x = -9/5 = -1.8
y = -7 - 2x = -7 + 2*1.8 = -3.4
2)y+3x=11
+
2x-y=9 

5x = 20
x = 4
y = 11 - x = 11 - 3*4 = 11 - 12 = -1
3)x+y=3
2x-3y=-4
x = 3 - y
2(3 - y) - 3y = -4
6 - 2y - 3y = -4
-5y = -10
y = 2
x = 3 - 2 = 1
4)7x-15y=129|*3
 -3x-11y=-3|*7
21x - 45y =  387
+
-21x - 77y = -21

-122y = 366
y = -3
x = (-3 + 11y)/-3 = (-3 + 33)/-3  = 30/-3 = -10