$PV = nRT$

$P^3 V = k $ (a constant) $ \rightarrow P = (kV^{-1})^{\large\frac{1}{3}}$

$\Rightarrow (kV^{-1})^{\large\frac{1}{3}} V = nRT$

$\Rightarrow k^{\large\frac{1}{3}} V^{\large\frac{2}{3}} = nRT$

Differentiating, we get, $ k^{\large\frac{1}{3}} \large\frac{2}{3}$$ V^{\large\frac{1}{3}} dV = nRdT$

$\Rightarrow \large\frac{2}{3}$$P(\Delta V) = nR \Delta T$

Since $\Delta V$ is positive, $\Delta T$ is positive, and hence internal energy increases (i.e, it has a positive $\Delta$).