= Solution
For fixed <cosmological density parameters>, the <redshift drift> has the form $\dot z=H_0F(z)$. Multiplying the <Hubble constant> changes the vertical scale of all three sketches but leaves their zeros unchanged. Thus \b[the drift-zero <redshift> is independent of $H_0$ at fixed density fractions]. Every model has the trivial $z=0$ zero; only the specified mixed model has a further positive zero, $z_*\simeq2.089$. If instead physical matter and vacuum densities are held fixed while $H_0$ changes, their density fractions change too, so that is a different family of models.
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