Conic Sections · Superchallenge problem

Point $P$ moves along the line $3x + 4y = 50$. From $P$, two tangent lines are drawn to the circle $x^2 + y^2 - 6x - 8y + 21 = 0$, touching it at $A$ and $B$. Let $C$ be the centre of the circle. Find the smallest possible area of quadrilateral $PACB$. Give your answer in simplest radical form.