The atomic number of strontium is 38 according to the Element Tables. The atomic number of uranium is 92. So the second nucleus on the right must have atomic number 54 (92 - 38).
The Element Tables show that element number 54 is xenon (Xe).
To find the number of free neutrons, compare the mass numbers on right and left. Uranium is 236 on the left, strontium and xenon are 90 + 141 = 231. The difference is 236 - 231 = 5. So there are 5 free neutrons. (Since the proton numbers are known to be equal, the difference in mass number must be the difference in neutron number.)]
Alternately: In the isotope 141Xe, there are 87 neutrons (87 = 141 - 54). So the number of neutrons in the nuclei are:
A total of 139 neutrons on the right (52 + 87), 144 neutrons in the uranium; 5 neutrons left over.
The equation is: