1)
y = x+1
x^2 + 2y =1
x = y-1
(y-1)^2 +2y = 1
y^2 -2y + 1 + 2y = 1
y^2 = 0
y = 0
x = -1
2)
x=2y
x^2-y^2 = 3
x^2-y^2 = (x-y)(x+y) = y * 3y = 3
y^2 = 1
y = 1
x = 2
3)
x-y = 7
xy = 18
x = 7+y
xy = (7+y)y = 7y+y^2 = 18
y^2 +7y - 18 = 0
D = 49 + 72 = 121
y1 =(-7 + 11)/2 = 2
y2= (-7-11)/2 = -9
x1 = 7+y1 = 9
x2 = 7+y2 = -2