The sum of possible values of \( x \) for \( \tan^{-1}(x+1) + \cot^{-1}\!\left(\dfrac{1}{x-1}\right) = \tan^{-1}\!\left(\dfrac{8}{31}\right) \) is:
\( -\dfrac{30}{4} \)
\( -\dfrac{32}{4} \)
\( -\dfrac{33}{4} \)
\( -\dfrac{31}{4} \)