Let the line \( x-y=4 \) intersect the circle \( C:(x-4)^{2}+(y+3)^{2}=9 \) at the points \( Q \) and \( R \). If \( P(\alpha,\beta) \) is a point on \( C \) such that \( PQ=PR \), then \( (6\alpha+8\beta)^{2} \) is equal to __________.
