You need to solve the following equation: [itex]\Delta G = \Delta H  T \Delta S[/itex]. You know [itex]\Delta G[/itex] and [itex]T[/itex], which leaves two unknowns: [itex]\Delta H[/itex] and [itex]\Delta S[/itex].
You cannot solve for two unknowns with a single equation: you need to have two sets of values. The way to do this is to solve the following set of simultaneous equations, using the value of [itex]\Delta G[/itex] and two different temperatures.
[itex]\Delta G_{(1)} = \Delta H  T_{(1)} \Delta S[/itex]
[itex]\Delta G_{(2)} = \Delta H  T_{(2)} \Delta S[/itex]
The difference between these two [itex]\Delta G[/itex] values isn't negligible [itex]\Delta G[/itex] is a temperature dependent quantity!
