PROBLEM SET 6, QUESTION 2:

The number of neutrons in the isotopes are:

uranium, 144 (236 - 92)
bromine, 53 (88 - 35)
lanthanum, 83 (140 - 57)

So the number of neutrons in the two isotopes on the right is 53 + 83 = 136. There are 144 neutrons in the uranium nucleus. So there are 8 neutrons left over (144 - 136).

Alternately, just calculate the mass numbers, which are the total of protons plus neutrons. On the right there is 140 + 88 = 228. Compare this to mass number 236 on the left. The difference is 8.