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

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 ca开发者_如何学运维n 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

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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.

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Though Will's answer is the canonical way to do it, I'll provide a few alternatives just for fun.

In[37]:= ans={{b -> -1, t -> 0},{b -> 0, t -> 0},{b -> 1, t -> 2},{b -> 2, t -> 0}};

In[38]:= Cases[ans, (b -> a_) :> a, Infinity]

Out[38]= {-1, 0, 1, 2}

In[39]:= ans[[All, 1]][[All, 2]]

Out[39]= {-1, 0, 1, 2}

In[40]:= ans /. {b -> a_, _} :> a

Out[40]= {-1, 0, 1, 2}

In[41]:= (ans /. Rule -> List)[[All, 1, 2]]

Out[41]= {-1, 0, 1, 2}