Let the abscissa of the two points $P$ and $Q$ on a circle be the roots of $x^2-4x-6=0$  and the ordinates of $P$ and $Q$ be the roots of $y^2+2y-7=0$ . If $PQ$ is a diameter of the circle $x^2+y^2+2ax+2by+c=0$ , then the value of $\left(a+b-c\right)$ is

Let point P(h, k) & Q(p, q)
Equation of circle by using diameter form
(x – h) (x – p) + (y – k) (y – q) = 0
(where h, p are the roots of $x^2-4x-6=0$ and k, q are the roots of  $y^2+2x-7=0$
$x^2+y^2-4x+2y-13=0$
Now, Compare it with the given equation, we get
a = -2, b = 1, c = -13, Now, a + b – c = 12