At room temperature and ordinary pressure, graphite is the more stable form of carbon by Δ G =…

Step 1. Molar volumes. Graphite: $12.011 / 2.26 = 5.31\ \text{cm}^3$. Diamond: $12.011 / 3.51 = 3.42\ \text{cm}^3$.

Step 2. The change. $\Delta V = 3.42 - 5.31 = -1.893\ \text{cm}^3 = -1.893 \times 10^{-6}\ \text{m}^3$.

Step 3. Equilibrium. $\Delta G + p\,\Delta V = 0$, so $p = -\Delta G / \Delta V = 2900\ \text{J} / (1.893 \times 10^{-6}\ \text{m}^3) = 1.53 \times 10^9\ \text{Pa}$.

Source. The Gibbs energy of formation of diamond, $+2.90\ \text{kJ/mol}$, is a standard value from the NIST tables and the CRC Handbook.