Thus 340 J heat corresponds to 0.091 ml of volume contraction.

The volume contraction of 1.82 ml correspond to $\frac{340}{0.091}\times 1.82=6800$ J or 6.8 kJ .

Since combustion is an exothermic reaction, there is decrease in the internal energy. Due to this, the change in internal energy has negative sign. Hence, the internal energy change of combustion for methanol is –6.8kJ/mol.