(a)
$2.\overline{357} = 2 + 0.357 + 0.000357 + ...$
$\Rightarrow \ 2.\overline{357} = 2 + \frac{357}{10^{3}} + \frac{357}{10^{6}} + ...$
$\Rightarrow \ 2.\overline{357} = 2 + \frac{\frac{357}{10^{3}}}{1 - \frac{1}{10^{3}}} = 2 + \frac{357}{999} = \frac{2355}{999}$