I have this problem here which I can't seem to solve (why is my intro always like this):

$\displaystyle \sqrt{(x^2+y^2-20)^2}+[x^2+y^2+xy-18]=0$

The expression inside the [ ] means the absolute value. I can't seem to express absolute values, so.

When real number x and y satisfies the above equation, what's the value of

$\displaystyle x^2-y^2$?

$\displaystyle (x < y < 0)$

Can anyone help me out here?