How many atoms is 87.4 grams potassium?

1.34565x10^24 atoms

You set up a conversion factor. Since 39.1 is the atomic mass of potassium,

87.4g K (1 mol/39.1g) = 2.2353mol

Since 1 mol has 6.02x10^23 atoms, you can use another conversion factor to find atoms

2.2353mol K (6.02x10^23 / 1 mol) = 1.34565x10^24 atoms