urgent substitution

bonjour,

x+y = 18x²
x²+y² = 1800
x =-y+18

(-y+18)²+y² = 180
y²-36y+324+y²-180 = 0
2y²-36y+144 = 0

b²-4ac = (-36)²-4(2*144) = 144
Δ = 12

-b-√Δ/2a = 36-12/4 = 6
-b+√/2a = 36+12/4 = 12