To find the number of neutrons in an atom, you would subtract
a.) mass number from atomic number
b.) atomic number from mass number
c.) atomic number from electron number
d.) isotope number from atomic number

To find the number of neutrons in an atom, what we must do is to follow the following steps:

1) Find the element in the periodic table.

2) Find the atomic number of the element. The atomic number is the number of protons of a single atom of that element.

3) Find the atomic weight of the element.

4) Finally, we look for the number of neutrons. For this, we follow the following relationship:

Atomic weight - number of protons = number of neutrons.

To find the number of neutrons in an atom, you would subtract:

B.) atomic number from mass number

your answer is, orbit around the nucleus