Relativity & Cosmology Problem No. 1

Problem No.1 (Relativity & Cosmology) A car of mass 2000 kg moving with a speed of 20.0 m/s collides and locks together with a 1500-kg car at rest at a stop sign. Show that momentum is conserved in a reference frame moving at 10.0 m/s in the direction of the moving car. Reference PHYSICS for Scientists and Engineers with Modern Physics Raymond A. Serway John W. Jewett, Jr. Cengage 9th Edition (2014) Ch. No. 39 (Relativity) Exercise No. 39.4