Background Info:
The standard enthalpy of formation (ΔH∘f) is the enthalpy change that occurs when exactly1 mol of a compound is formed from its constituent elements under standard conditions. The standard conditions are 1 atm pressure, a temperature of 25 ∘C , and all the species present at a concentration of 1 M . A "standard enthalpies of formation table" containing ΔH∘f values might look something like this:SubstanceΔH∘fH(g)218 kJ/molH2(g)0 kJ/molBa(s)0 kJ/molBa2+(aq)−538.4 kJ/molC(g)71 kJ/molC(s)0 kJ/molN(g)473 kJ/molO2(g)0 kJ/molO(g)249 kJ/molS2(g)129 kJ/mol

Question:
What is the balanced chemical equation for the reaction used to calculate ΔH∘f of BaCO3(s)?If fractional coefficients are required, enter them as a fraction (i.e. 1/3). Indicate the physical states using the abbreviation (s), (l), or (g) for solid, liquid, or gas, respectively. Use (aq) for aqueous solution.
EXPRESS ANSWER AS A CHEMICAL EQUATION. Please explain for me too!!!