**This is a physics question**

Nuclide Pu is a alpha emitter. Pu(A) = 239, Pu(Z) = 94

a) Write the formula for the decay and calculate the energy per decay.

I have the formula done

Pu -> U + He | U(A) = 235, U(Z) = 92 , He(A) = 4, He(Z) = 2

Now I have to sum up the nuclide masses for He-4 and U-235 which is

4.002603u + 235.0439u = 239.0465u.

Now I am stuck, according to my solution booklet I should do this:

239.0522u (the Pu) - 239.0439 (where do this value come from?) = 0.0057u

Even if I put this 239.0522u - 239.0439u in my calculator I do not get 0.0057u I get 0.0083.

So where did 239.0439u and 0.0057 came from? I have read in the book on the chapter about this but it still doesn't makes any sense to me