Find a, b, and c
number of protons=a
number of neutrons=7
atomic number=b
mass number=15
element symbol=c
~ provide explanation~

Electrons = protons
protons = atomic number
protons + neutrons = atomic mass

Therefore: a) protons + 7 = 15
                        protons= 15 -7
                        protons= 8

B) Remember proton = atomic number
     so atomic number = 8

C)Now we know the atomic number so we can easily check on the periodic table what element has an atomic number of 8, which is Oxygen