In $\triangle A B C$, $a = 3 , \quad b - c = 2 , \quad \cos B = - \frac { 1 } { 2 }$. (I) Find the values of $b$ and $c$; (II) Find the value of $\sin ( B - C )$.
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In $\triangle A B C$, $a = 3 , \quad b - c = 2 , \quad \cos B = - \frac { 1 } { 2 }$.
(I) Find the values of $b$ and $c$;
(II) Find the value of $\sin ( B - C )$.