Enthalpy change of solution

How can I calculate the enthalpy change of solution from heat energy absorbed in a reaction ... E.g. "2.42g of potassium chloride are dissolved in 44.96g of water. The temperature of the water falls from 22.6 to 19.9(degrees celcius). A) calculate heat energy absorbed in this reaction = 507.4
B) hence calculate the enthalpy change of solution for potassium chloride in KJmol^-1 ?

is the answer -15.6 kg mol-1?

No, it's +15.6 kJ mol^-1

Energy change is given by E = mcΔT
Then divide by the number of moles of solute used to give energy change per mol.