Hello.
In a molecule, one mole of the molecule contains Avagadro's number of molecules.
So we can safely say that 0.788 moles of NaOH contain 0.788*Av. (Where Av is Avagadro's number)
And in a molecule of NaOH there are 3 atoms, so we multiply the number of molecules by 3 to obtain the number of atoms
3x0.788xAv
=3x0.788x6.02x10^23
=1.42x10^24 Atoms.
Have a nice day
