2 different steps here

1. Need to find

Heat absorbed by the calorimeter = Heat capacity x delta T

2. Heat absorbed by the calorimeter is the same #joules as the heat given off by the combustion (they are actually opposite in sign, one is + the other is -) but for this use the value as +

a) Heat of combustion (for 2.839 g) = answer from 1

b) how many moles in 2.839 g ?

Calculate heat of combustion PER mole using a) and b) (what's the math?)