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 (α, β) is a point on C such that PQ=PR, then (6 α+8 β)^2 is…

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