Answer:
141 J Â
Explanation:
The lowest sig digits in a calculation determine the sig dig of the answer
The temps are listed with 3 significant digits and the spec heat is 3 sig digits  (the mass has 4 sig digs) .....so answer will need 3 digits
31.91 g  * .903 J/g-C  *  (26.1 - 21.2 C) = 141 J  (three sig dig)