= Solution
Subtract reaction (b) from reaction (a) and divide by two. In the direction chosen here, the resulting balanced conversion is
$$
\mathrm{NaAlSiO_4}+2\mathrm{H_4SiO_4^0}
=\mathrm{NaAlSi_3O_8}+4\mathrm{H_2O}.
$$
Thus <nepheline> plus dissolved silica converts to <albite>. Additivity of the <Gibbs energy of reaction>, equivalently <Hess's law>, gives
$$
\boxed{\log_{10}K_c=\tfrac12(\log K_a-\log K_b)
=\tfrac12(6.88+15.18)=11.03.}
$$
For the reverse reaction the answer is $-11.03$; the reaction direction must accompany the <equilibrium constant>.
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