Galena is the ore from which elemental lead is extracted. in the first step of the extraction process, galena is heated in air to form lead(ii) oxide. 2pbs(s) + 3o2(g)  2pbo(s) + 2so2(g) h = –827.4 kj what mass of galena is converted to lead oxide if 975 kj of heat are liberated

To illustrate clearly, I will rewrite the reaction in a more understandable manner.

2PbS (s) + 3 O₂(g) ⇒  2 PbO(s) + 2 SO₂ (g)              Δh = –827.4 kJ

Galena is PbS and Lead Oxide is PbO. This reaction has a negative sign for the change in enthalpy of reaction. The sign convention only means that the reaction releases energy to the surroundings. In other words, the reaction is exothermic. Focusing on only its magnitude, this means that 827.4 kJ of energy is needed for this reaction of 2 moles of Galena and 3 mole of O₂  to occur. 

Now, if you had an energy of 975 kJ, then the reaction would proceed into completion and would even exceed in the number of moles of products produced. We use stoichiometric calculations as follows:

975 kJ * (2 mol PbS/827.4 kJ) = 2.357 moles of PbS

Knowing that the molar mass of Galena is 239.3 g/mol, then the mass of galena needed to produce PbO is 

2.357 mol PbS *239.3 g/mol = 564 g galena