A substance consists of atoms which all have the same number of protons and electrons but different numbers of neutrons. Why is this substance an element?
Chemical elements are defined by the number of protons,.......
But the atomic nucleus can contain DIFFERENT numbers of neutrally charged, massive particles, i.e. NEUTRONS, and these neutrons give rise to the phenomenon of
Most hydrogen nuclei contain just the one particle, the one proton, and this defines the atom as a hydrogen atom, i.e.
Back to your problem (finally!), the question specified that your atoms contained the same number of protons (and the neutral atom must necessarily contain the same number of electrons, why?). Thus, BY DEFINITION, if they contain different numbers of neutrons, THEY ARE ISOTOPES of the same element.