Chemical energetics: explain question

n-Butane, $\ce{C4H10}$, can be converted into its branched-chain isomer 2-methylpropane, $\ce{C4H10}$, using a suitable catalyst. The enthalpy change of this isomerisation is difficult to measure directly because the reaction is slow and the heat change is very small. The standard enthalpy changes of combustion of the two isomers, however, can be measured easily.

Explain how the standard enthalpy changes of combustion of the two isomers could be used to obtain the enthalpy change for the isomerisation of n-butane to 2-methylpropane, and why the method is valid.