Ok, first part of the question works out 1.00mol of nitromethane burned in oxygen forms total volume of 7.43x10-2 m3 of gases at 298K and 100KPa.
v= (3.00mol x 8.31J K-1 mol-1 x 298K) / 100000KPa = 7.43x10-2 m3

The equation is:
2CH3NO2(l) 11/2O2(g) > 2CO2(g) 3H2O(g) N2(g)

Now it says: The combustion reaction is very exothermic and reaches 1000k. Determine the total volume of gases when the temp is raised to 1000k at a constant pressure.

Any clues what I am supposed to do here. Am I supposed to just substitute values in ideal gas eq. with 1000K and 6mol?

No no, you use 3 mol and 1000 K.

You didn't use 6 mol in the initial problem, why would you use 6 mol now?