Find the joint equation of lines $x+y+1=0$ & $x+y-1=0$

Question

$\begin{array}{1 1}(a)\;x^2+y^2+xy=1&(b)\;x^2+y^2+2xy=1\\(c)\;x^2+2y^2+xy=3&(d)\;x^2+y^2+3xy=1\end{array}$