a
(a) \(\mathop {{H_2}{S_2}{O_7}}\limits^{ * \,} \)

\(2 \times ( + 1) + 2 \times x + 7 \times ( - 2) = 0\)

\( + 2 + 2x - 14 = \)\(2x = 14 - 2 = 12\)

\(x = \frac{{12}}{2} = + 6\,\,{\rm{for}}\,\,S\)

\({K_4}\mathop {Fe}\limits^{ * \,\,} {(CN)_6}\)

\(4 \times ( + 1)\,x + 6 \times ( - 1) = \)\(4 + x - 6 = 0\)

\(x = 6 - 4 = + 2\,\,{\rm{for}}\,\,Fe\).