1) Write the symbol and charge for each individual ion 2) Balance the charge so that they equal to zero
3)Write the formula based on how many of each ion you used in step 2

11) Beryllium and Phosphorus
12) cesium and Sulfur
13) Lithium and phosphorus
14) fluorine and rubidium
15) Francium and oxygen

I'm pretty sure these are correct

11) Be 2+ and P 3-    Formula is Be3P2  

12) Cs 1+ and S 2-    Formula: Cs2S

13) Li 1+ and P 3-      Formula: Li3P

14) Rb 1+ and F 1-     Formula: RbF

15) Fr 1+ and O 2-    Formula: Fr2O

Explanation:

You do the criss-cross method where the charge on one ion becomes the amount on the second

i'm wiling to try and you : )