Ihave an atom of an unknown element. it has a mass or 57 with 30 neutrons. this must be an isotope of what element

The unknown element must be an isotope of Cobalt. 
This is because cobalt has an atomic number of 27. The mass number of an atom is the summation of its proton and neutrons. For the given unknown element, the mass number is 57 and the neutron number is 30, which means that 27 protons are present. The number of electrons and proton inside an atom is always the same, so the unknown atom has 27 electrons which means that is atomic number is 27.
Thus, the unknown element has 27 electrons, 27 protons and 30 neutrons instead of 27 neutrons, this shows that is an isotope of cobalt.