Calculate the number of oxygen atoms in 2.00 moles of Mn2O7

First you should know that there is seven oxygen atoms in one Mn2O7

So

2.00 moles of Mn2O7 contain 14.00 moles of oxygen ...

Then you multiply this no. with Avagadro no ...

from formula

Number of moles = no. of particles/avagadro's no ...

14.00*6.02*10²³=84.28 atoms of oxygen ...