That's not the correct line.
The calorimeter is a bit slow in detecting the temperature rise, so there is a theoretical start of the process exactly vertically up (where the green line is) that is missed by the detector. That will line up with your final temperature.
You were supposed to extrapolate using the initial drop (your red) from
As such, your final temperature was supposed to be around
Therefore, your change in temperature, assuming an ideal temperature increase, would be:
#26.25^@ "C" - 23^@ "C" ~~ color(blue)(3.25^@ "C")#.
With this info, that's all we can do. Sorry, you needed to provide a lot more info, like the mass of your materials, your volume of water, etc...
Besides that, we don't even know what time units you have in your