From your T vs. t plots, determine ΔT for the reaction by extrapolating the temperature lines before and after reaction towards the moment of mixing. This calculation can be done with some creative application of slope and intercept functions in Excel. This initial step is not so difficult - determining the error in these numbers is much more taxing. Use the handout axamples and the various Excel functionalities to assess the error as best you can. If you have questions as to how to go about this, feel to ask.