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