Um, I'm afraid you got the wrong procedure. The mass of each element will not be an exact multiple of u, since there is some of that mass conserved into binding energy.

What you need to do, is find the difference in binding energies.

Binding energy in U-235 = 235*7.2 = ?

Binding energy in Cs-142 + Rb-94 = 8.4*142 + 8.6*94 = ?

Then work out the difference. Since the products are more stable, you'll have them having greater binding energy. The difference is the energy released.

Remember that:

Relative atomic mass = Total mass of each nucleon - Binding energy

Hence why the energy released will appear to be negative.

And Dan, the reaction is ideally:

U-235 + neutron ---> Rb-94 + Cs-142

That's where the extra neutron comes from.