Explanation :

Molar heat of sublimation is the energy required to change one mole of a solid directly into a gas at a constant temperature and pressure. 

This process involves two steps: first, the solid undergoes a phase change from solid to liquid (fusion), and then from liquid to gas (vaporization). 

Therefore,

The molar heat of sublimation is equal to the sum of the molar heats of fusion and vaporization.