Hi Husen,
Please not that, t-1 and -t-1 add up to -2.
So sum of roots = -2 = (-2a)/(a+2)
or, a/(a+2) = 1............... (which can never be possible for any real value of a)
Hence there can be no value of 'a' for which t-1, -t-1 are roots of that Quadratic Equation.
Hope that was helpful.
All the Best.
Best Regards,
Ashwin (IIT Madras)