So in words basically, as it is a perfectly elastic collision, no kinetic energy is loss. Therefore the momentum before the collision equals momentum after the collision (conservation of momentum). We can then calculate the momentum before the collision by multiplying the mass of the proton to its velocity (don't worry about the unit of the mass of the proton because it'll cancel out with the unit of the mass of the carbon atom), and then divide it by the sum of the mass of carbon atom and the mass of proton to get the velocity of both particles travelling in the same direction (to the right, as the symbol doesn't change). I don't know whether your question has something special (like some stuff in A-levels) or not but mine uses only GCSE knowledge.