hypev Y=4-x
x^2+3xy=18

y=4-x
x^2+12x-3x^2=18

-2x^2+12x-18=0
x^2-6x+9=0
D=b^2-4ac=36 - 4×1×9=36-36=0
x=-b/2a=-(-6/2×1)=3

x=3
y=4-x=4-3=1

(3;1)