A clay ball with a mass of 0.35 kg has an initial speed of 4.2m/s. It strikes a 3.5kg clay ball at rest, and the two balls stick together and remain stationary. What is the decrease in kinetic energy of the 0.35 kg ball?

Kinetic energy is 1/2mv^2. If the 0.35Kg ball becomes stationary then its velocity is 0, hence it's KE is 0. Can you take it from here?