Barium is in Group 2, so we know that it makes 2+ ions. If you happen to have a chart of common polyatomic ions, you can find out that phosphate is PO
4 and has a 3- charge. So, to even out the charges, we need to get three barium ions and two phosphate ions. Now, the total positive charge (from the barium ions) is 6+, and the total negative charge (from the phosphate ions) is 6-.
We see that
Ba3(PO4)2 is the chemical formula for barium phosphate.