Mathematica: Getting rid of the "x ->" in FindInstance results

Question

Suppose I have the following results:

a=FindInstance[2*b^2 + b^3 == b^4 + t && t < 10 && t > -1, {b, t}, 
  Integers, 20]
{{b -> -1, t -> 0}, {b -> 0, t -> 0}, {b -> 1, t -> 2}, {b -> 2, 
  t -> 0}}

How can I get rid of the "b->" and just get the array of b answers? I can get halfway there with:

a[[All,1]]
{b -> -1, b -> 0, b -> 1, b -> 2}

but how can I get to just:

{-1, 0, 1, 2}

Thanks

Answer 1

I might be missing something from dreeves' answer, but the way I always believed you do this was simply by writing:

b /. a

There is an example of this in the "Basic Examples" section of the documentation for the Solve function, which uses the same output style.