How many neutrons does element x have if it’s atomic number is 40 and it’s mass number is 18

I believe you may have flipped which numbers apply where. Since each proton and neutron equal a mass of 1 amu (atomic mass units), the mass cannot be lower than the atomic number.

So if the atomic number is 18, and the atomic mass is 40, that means that the number of neutrons is the difference.

so, you have

each neutron weighs 1 amu so you should have 22 neutrons.

22 Neutrons

a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.