Hi
If you assign numerical values to the 7 parameters
x1, x2, x3, b, c1, c2, c3
then you get a result very quickly.
I just tried that with
{x1, x2, x3, b, c1, c2, c3} = RandomReal[{-3, 3}, 7]
(x1
a real number in the range {-3,3}
, x2
a real number in the same range, etc.
It looks like a too complicated transcendental equation for a2
, so as I see it you have to work with numerical parameters, not symbolic parameters.