What is the specific heat of aluminum if the temperature of a 28.4 g sample of aluminum is increased by 8.1 C when 207 J of heat is added?

The working is

207 / (28.4/1000) * 8.1

(you divide the 28.4 by 1000 to turn it into kgs)

this is because energy (J) is equal to mass times specific heat times temp. difference.

therefore specific heat will be equal to energy divided by mass times temp. difference.