How many neutrons does X have if it's atomic number is 28 and it's mass number is 71

We know that the mass number is equal to the number of protons plus the number of neutrons. We also know that the atomic number is the same number as the number of protons. Therefore, we can solve for the number of neutrons with what we were given:

x + 28 = 71

x=43

There are 43 neutrons