There are 3 Barium, 2 Phosphorus and 8 Oxygen in Barium Phosphate (Ba3(PO4)2, making 13 units. Multiply 0.0350 mol given by 13 and then you get 0.455 mol. Since they want atoms, multiply 0.455 by Avogadro's number (0.455 x 6.023 x 1023) and you get 2.74 atoms of barium phosphate.
