$$x(1-y^2)dx+y(1-x^2)dy=0\Rightarrow \frac{x}{1-x^2}dx+\frac{y}{1-y^2}dy=0$$
$$\Rightarrow$$
$$\frac{2x}{x^2-1}dx+\frac{2y}{y^2-1}dy=0$$
$$\Rightarrow$$
$$\frac{2x}{x^2-1}dx+\frac{2y}{y^2-1}dy=d(c)$$
$$\Rightarrow$$
$$\int \frac{2x}{x^2-1}dx+\int\frac{2y}{y^2-1}dy=\int d(\ln c)$$
$$\Rightarrow$$
$$\ln (x^2-1)+\ln (y^2-1) =\ln c$$
$$\Rightarrow$$
$$\ln [(x^2-1)\cdot (y^2-1)] =\ln c$$
$$\Rightarrow$$
$$(x^2-1)\cdot (y^2-1) =c$$