Your question is asking for the heat of formation of UO3 per mole O2(g) consumed
You have to find the value of enthalpy of formation since the reaction is the standard heat of information. The value is -1223.8. This represents the amount of heat released when 1 mol of UO3 is formed. Now 1 mol of UO3 needs 3/2 moles of O2(g).
The equation will look like this
-1223.8kJoules / molUO3 x 2molO2 = -815.7 kJ
3mol uO3 molO2 (g)