double equation

Well,
x=6-y (from the first equation)
Substitute this into the second equation and it will all fall out nicely.
If that is not enough help just ask again.

x=6-y

(6-y)^2-y^2=12

36-12-y^2+y^2-y^2=12

36-12-y^2=12

-y^2=12-36+12

-y^2=-12

y=sqrt(12) <-- Which isn't correct

P.S. Yesterday i tried to solve it like 10 times in various ways it took me about 5mins (which never takes me so long), but i didn't figure out it.

[input]solve( x+y=6, x^2-y^2=12 )[/input]

x+y=6
x=6-y

x^2-y^2=12
(6-y)^2-y^2=12
36-2*6*y+y^2-x^2=12
36-12y=12
-12y=-24
y=24/12
y=2

x=6-y=6-2=4
x=4

So i did fail at "squaring", thank you for pointing me out.