Suppose the mass spectrum of a hypothetical monatomic element x contains a signal at mass number 13 and another of identical height at mass number 15. a) sketch the mass spectrum. make sure each axis is properly labeled b)how many isotopes are present. why

Although this is not my field, I think I can help you. 

a) Your graph should have two vertical lines, of equal height at mass 13 and 15 respectively. 
b) Only 1 isotope would be present, i.e. the principal element with mass M=13 and then one isotope at mass M+2 

We are assuming that the principal element is the one that is the lowest mass - by definition, an isotope is one where there are additional neutrons - hence the mass increases, but the proton count is the same. Having an equal height means that the relative abundance of both are the same. 

D. is the answer hope that you