Sr will always have 38 protons because that is its atomic number. Normally Sr has 38 protons and 49 neutrons yielding an atomic mass of 87. unless you are dealing with an isotope of Strontium in which 3 additional neutrons would give you the 53 neutrons in your answer, (38p, 52n).
By adding together the number of protons and neutrons and multiplying by 1 amu, you can calculate the mass of the atom.
17 protons 17 electrons 18 neutrons
