A circle with centre \( (2, 3) \) and radius \( 4 \) intersects the line \( x + y = 3 \) at the points \( P \) and \( Q \). If the tangents at \( P \) and \( Q \) intersect at the point \( S(\alpha, \beta) \), then \( 4\alpha - 7\beta \) is equal to _________.
