= Solution
Differentiate the <equation of state> at fixed <temperature>:
$$
\chi=\frac1{a+3bm^2}.
$$
Above the transition, zero-field equilibrium has $m=0$. Below it the stable pure phases have $m^2=-a/b$. Thus
$$
\boxed{\chi(t)=\frac1{a_0t}\ (t>0),\qquad
\chi(t)=\frac1{2a_0|t|}\ (t<0),\qquad
\lim_{t\to0^+}\frac{\chi(t)}{\chi(-t)}=2.}
$$
This is the <mean-field magnetic-susceptibility amplitude ratio>. Below the transition, take a one-sided response in a selected ordered phase: the globally minimizing <magnetization> switches discontinuously between opposite phases when $h$ crosses zero. Its two-sided <derivative> at precisely zero field is therefore not the finite pure-phase <magnetic susceptibility> used in the ratio.
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