18х+у=-2
у=-2-18х
-----------------
2х-у=11
у=2х-11
-----------------
0,7х+4у=11
4у=11-0,7х
у=(11-0,7х)÷4
-----------------
15х-25у=0
25у=-15х
25у=15х
у=15х÷25
(x=7+y
(X^2+2xy+y^2-9=0
(7+y)^2+2y(7=y)+y^2=0
49+14y+y^2+14y+2y^2-9=0/4
Y^2+7Y+10=0
D=49-40=9
Y=-7+3÷2=-2
Y=-7-3÷2=-5
y=-2 x=5
y=-5 x=2
x^4+6x^3+9x^2-14x^2-42x+40=0