determine the region where product <0

We have 2 functions
$f_{1}(x,y,z)=A_{1}xyz+B_{1}xy+C_{1}xz+D_{1}yz+E_{1 }x+G_{1}y+
M_{1}z+N_{1}$

$f_{2}(x,y,z)=A_{2}xyz+B_{2}xy+C_{2}xz+D_{2}yz+E_{2 }x+G_{2}y+
M_{2}z+N_{2}$

So i need to find where $f_{1}*f_{2}<0$
Any ideas or mb u know where i can find solution?