Tolong dibantu ya,,,aku gak paham

Z A
X
keterangan:
X = lambang atom
Z = nomor atom = Jmlh Proton = jmlh elektron
A = nomor massa = proton + neutron
neutron = nomor massa – nomor atom
ex:
11 23
Na
proton = 11
elektron=11
neutron=23-11=12