= Solution
The two <binodal> compositions $\phi_1<0<\phi_2$ are the contact points of the <common-tangent construction>. Equivalently, they solve
$$
f'(\phi_1)=f'(\phi_2)=\mu,
\qquad
f(\phi_1)-\mu\phi_1=f(\phi_2)-\mu\phi_2.
$$
The first equality is equality of <chemical potential>; the second is equality of <pressure>. Here
$$
(a-C)\phi_1+b\phi_1^3=a\phi_2+b\phi_2^3
$$
together with the intercept equation determines the two densities. As $C\downarrow a$, both contact points approach zero continuously with scale $|\phi_{1,2}|\propto\sqrt{C-a}$, so this mean-field onset of phase separation is continuous.
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