The enthalpy change when the substance is dissolved is reflected in the heat released when that is done. The heat released raises the tempereature of the water.
Here, 2 gram of a substance with a molar mass of 56 is dissolved in 28 g of water. The result is an increase in the temperature of water from 12 degree Celsius to 27 degree Celsius.
2 gram of the substance is equal to 2/56 moles. The heat capacity of water is 4.1855 [J/(g·K)]. To raise the temperature of 28 g of water from 12 degree Celsius to 27 degree Celsius requires (27 - 12)*28*4.2 = 1764 J
For 1/28 mole of the substance the enthalpy change is 1764 J. For 1 mole it is 1764*28 = 49392 J
The enthalpy change per mole when the substance is dissolved is 49.392 kJ/mol