Either light is an exception or the classical velocity addition formula only works at low velocities. The correct formula for one-dimensional relativistic velocity addition is u dfracv u1 fracvuc2. In the equation the increased relativistic mass m of a body times the speed of light squared c2 is equal to the kinetic energy E of that body.
Light - Light - Reflection and refraction.
However the usual approach is setting x0ct and so all the four coordinates have units of length. Enterprise set in 2151 and onwards follows the voyages of the first human ship capable of traveling at warp factor 52 which under the old warp table formula the cube of the warp factor times the speed of light is about 140 times the speed of light ie 52 cubed. The fact that cm is asked for in the problem allows us to use the cms value for the speed of light. G M1 L1 T-2 L2 M-2 M-1 L3 T-2.