After being produced in a collision between elementary particles, a positive pion (' +) must travel down

After being produced in a collision between elementary particles, a positive pion ('π +) must travel down a 1.20-km-Iong tube to reach an experimental area. A ' π + particle has an average lifetime (measured in its rest frame) of 260 x 10-8 s; the ' π + we are considering has this lifetime.
(a) How fast must the ' π + travel if it is not to decay before it reaches the end of the tube? (Since u will be very close to c, write u = (1 - ∆)c and give your answer in terms of ∆. rather than u.)
(b) The ' π+ has a rest energy of 139.6 MeV. What is the total energy of the π+ at the speed calculated in part (a)?