To answer this question, you need to recognize that energy is conserved. The initial energy was kinectic and then transferred to internal; therefore, in this problem, (aka final energy) internal energy = kinectic energy (aka initial energy).
Kinectic energy = 0.5 * m * v^2
5836 = 0.5 * m * (13.4)^2
89.78m = 5836
Mass = 65 kgs
