(b)
$K_{sp}\left\lbrack BaCO_{3} \right\rbrack = \left\lbrack Ba^{2 +} \right\rbrack\lbrack CO_{3}^{2 -}\rbrack$
$\left\lbrack Ba^{2 +} \right\rbrack = \frac{K_{sp}}{\lbrack CO_{3}^{2 -}\rbrack} = \frac{5.1 \times 10^{- 9}}{1 \times 10^{- 4}}$
$\left\lbrack Ba^{2 +} \right\rbrack = 5.1 \times 10^{- 5}\ M$