Let the abscissae of the two point \( P \) 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 \( (a + b - c) \) is ______.
