Find the equation of the tangent to each circle at the point specified

1 Circle $\displaystyle x^2 + y^2 - 8x - 42y = $ 0, point (8, 5)

2 Circle $\displaystyle x^2 + y^2 - 5x - 9y - 16 = 0 $ point(6, -4)

Any help would be great, don't even know what I should read up on to actually find out how to do it either...

Thanks