Answer: (3, -2)
Let the centre be C(x, y); CP = CQ = CR.
\(CQ^2=CR^2\): \((x-3)^2+(y+7)^2=(x-3)^2+(y-3)^2\) gives \(14y+49=-6y+9\), so \(y=-2\).
\(CP^2=CQ^2\): \((x-6)^2+(y+6)^2=(x-3)^2+(y+7)^2\) gives \(-12x+36+12y+36=-6x+9+14y+49\), i.e. \(-6x-2y=-14\), so \(3x+y=7\).
With \(y=-2\): \(3x=9\), \(x=3\). Centre (3, -2); radius \(=\sqrt{9+16}=5\).