If one of the diameters of the circle, given by the equation ( x^2 + y^2 - 4x + 6y - 12 = 0 ), is a chord of a circle, 'S', whose centre is at ( (-3, 2) ), then the length of radius of 'S' is ________ units.
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For a circle where a diameter is a chord of another circle, the distance between centres, radius of the first circle, and radius of the second circle form a right-angled triangle.