The reaction shown uses the notation for atomic number and mass number on the elements.
The bottom left number is the atomic number and it is related to which is the element. This number accounts just for the number of neutrons in the element.
The top left number is the mass number and it accounts for the number of particles that have considerable mass. This number accounts for the number of protons and neutrons.
We can't loss any particles in a nuclear reaction, so we have to count how many of each we have at start and at the end we must have the same.
At the start we have ₁²H and ₁³H.
The first, ₁²H, has mass number 2 and atomic number 1. The atomic number is the same as the number of protons, so it has 1 proton. the mass number is the sum of the number of protons and neutrons. Since we have mass number 2 and 1 proton, the other has to be a neutron.
So, there is 1 proton and 1 neutron.
The second, ₁³H, has mass number 3 and atomic number 1. So, the same number of protons, 1 proton, but now we have 2 neutrons, because we have 3 as mass number.
So, there is 1 proton and 2 neutrons.
At the end we have energy, which won't count any particles, we have an unknown particle, X, and ₀¹n.
The ₀¹n has atomic number 0, so there are no protons, that is, the mass number is all from neutrons. Since the mass number is one, we have 1 neutron. This is actually a single neutron, as noted by the "n".
Now, at the start we have:
1 proton + 1 proton = 2 protons
1 neutron + 2 neutrons = 3 neutrons
At the end we have 1 neutron and the rest must be all in X.
That is, all 2 protons have to be in X and 2 of the 3 starting neutrons must be in X.
So, X must have atomic number 2 (since it has 2 protons) and mass number 2 + 2 = 4 (since it has 2 protons and 2 neutrons).
Consulting a periodic table, we see that the element that has atomic number 2 is helium, He.
Thus, the particle is:
So:
Mass number: 4
Atomic number: 2
Chemical symbol: He