I have an atom of an unknown element. It has a mass or 57 with 30 neutrons. This must be an isotope of what element

The mass number of an atom is the summation of its proton and neutron.

Since the unknown atom has a mass of 57 and neutron number of 30, that means the number of proton is 27.

An element usually have the same number of proton and electron and this number represent the atomic number. The element with atomic number 27 is Cobalt.

Cobalt suppose to have 27 protons, 27 neutrons and 27 electrons. Thus the unknown atom with 30 neutrons is probably an isotope of cobalt.