It's important to know that fission reaction refers to the division of a nucleus in other lighter ones. In this case, we have nuclear fission because Pu is divided into two.
The element X must have a mass number of 103 and an atomic number of 35 because that's those are the number needed to make it even with Pu.
Therefore, the element is Lawrencium (Lr), it has a mass number of 103, an atomic number of 35, and its symbol is Lr.
