A neutral Kaon (K^0) is an unstable particle at a rest mass 8.87 * 10^-28 kg. One of the means by which it decays is by spontaneous creation of the two other particles called pions (pi^+ and pi^-), where the rest mass of each pion is approximately 2.49 * 10^-28 kg. The decaying process is given by: K^0 ---> pi^+ + pi^-
A kaon is moving in the +x direction decays by this process, where that pi^+ is moving off at a speed of 0.9c in the +x-direction and the pi^- is moving the opposite direction at a speed of 0.7c use the conservation of mass energy and momentum to determine the speed of the kaon before the decay.