19. The circle \(C\) has equation \(x ^ { 2 } + y ^ { 2 } - 8 x - 16 y - 209 = 0\).
- Find the coordinates of the centre of \(C\) and the radius of \(C\).
The point \(P ( x , y )\) lies on \(C\).
- Find, in terms of \(x\) and \(y\), the gradient of the tangent to \(C\) at \(P\).
- Hence or otherwise, find an equation of the tangent to \(C\) at the point (21, 8).