= Solution
Set $x=\log_{10}a_{\mathrm{H_4SiO_4^0}}$ and $y=\log_{10}(a_{\mathrm{Na^+}}/a_{\mathrm{H^+}})$, treating the supplied concentration notation as an ideal-dilute activity approximation. For pure solids and unit water activity, the first <chemical equilibrium> relation gives $\log K_a=2y+2x$ and the second gives $\log K_b=2y+6x$. Thus plot
$$
\boxed{y=3.44-x\quad(\text{nepheline–corundum}),}
$$
$$
\boxed{y=-7.59-3x\quad(\text{albite–corundum}).}
$$
Their <slopes> are $-1$ and $-3$ respectively. They intersect at \b[$(x_*,y_*)=(-5.515,8.955)$]. Both the relevant silica activities and the intersection lie at negative $x$, as expected for a dilute aqueous silica species.
\Image[/past-exam-of-the-year-2-of-the-earth-sciences-geology-course-of-the-university-of-oxford/2022/paper-1-mineral-activity.png]
{title=Original activity diagram: solid stable boundaries, dashed metastable continuations, and three mineral stability fields}
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