Circle

Let the abscissae of the two points \(P\) and \(Q\) on a circle be the roots of \(x^{2}-4 x-6=0\) and the ordinates of \(\mathrm{P}\) and \(\mathrm{Q}\) be the roots of \(y^{2}+2 y-7=0\). If \(\mathrm{PQ}\) is a diameter of the circle \(x^{2}+y^{2}+2 a x+2 b y+c=0\), then the value of \((a+b-c)\) is _____________.