P. Let's say the roots are $\alpha-d$, $\alpha$, $\alpha+d$. The sum of the roots is given by $-h$ (negative coefficient of $x^{2}$ over coefficient of $x^{3}$). So, we have $\alpha-d+\alpha+\alpha+d=-h$ which simplifies to $3\alpha=-h$ or $\alpha=-h/3$.
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