Answer:
The answer to your question is  C = 781.33 J/kg°K or
                               780 J/kg°K
Explanation:
Data
 mass = 250 g
 ΔT = 15 °K
 Q = 2930 J
 C =  J/kg°K
Formula
 Q = mCΔT
 Solve for C
 C = Q/mΔT
Process
1.- Convert g to kg
       1000 g -------------- 1 kg
        250 g -------------- x
        x = (250 x 1) / 1000
        x = 0.250 kg
2.- Substitution
  C = 2930 / ( 0.250 x 15)
3.- Simplification
  C = 2930 / 3,75
4.- Result
   C = 781.33 J/kg°K  ≈ 780 J/kg°K