x=7-5y
3x+8y=-21
3(7-5y)+8y=-21
21-15y+8y=-21
-7y=-42
y=-6
x=7-30
x=-23
ответ : (-23;-6)
х+5у=7
х+5=7
х=7-5
х=2
——————————
2+5у=7
x=7-5y
3x+8y=-21
x=7-5y
3(7-5y)+8y=-21
x=7-5y
21-15y+8y=-21
x=7-5y
-7y=-42
x=7-5y
y=-6
x=7-30
y=-6
x=-23
y=-6
ответ : (-23;-6)
х+5у=7
х+5=7
х=7-5
х=2
——————————
2+5у=7