The line \(y = 2x + 5\) intersects the circle with equation \(x^2 + y^2 = 20\) at \(A\) and \(B\).

(a) Find the coordinates of \(A\) and \(B\) in surd form and hence find the exact length of the chord \(AB\).

A straight line through the point \((10, 0)\) with gradient \(m\) is a tangent to the circle.

(b) Find the two possible values of \(m\).