According to einstein's equation, if 3 kg of mass "disappears" during a reaction, then

If 3 kg of mass "disappears" during a reaction, then energy "appears" and is equal to 
     E = mc²
     E = 3 kg * (3x10^8 m/s^2)²
     E = 27x10^16 Joules
The reaction converts mass into energy since they are equivalent as a conclusion of Special Relativity, the expression E = mc². The equation expresses a relationship between energy and mass that even small mass can be transformed into immense amounts of energy. 

Elias is almost correct. he just got it backwards.the correct answer is that it demonstrates elastic potential energy transforming into kinetic energy.