In any reaction (except for beta decay) the total number of neutrons is the same before and after, and the total number of protons is also the same before and after. In this process, the 21H has one proton and one neutron; the 31H has one proton and two neutrons. (Remember, the superscript on each nucleus is its mass number, or the sum of its protons and neutrons.) Thus we start with a total of 2 protons and 3 neutrons. After the reaction there is an alpha particle, 42He and an unknown particle, X. The alpha particle has 2 protons and 2 neutrons, so all that remains is one neutron. So X is that neutron.