Can someone please help me with percent yield?

Percent yield = actual yield/theoretical yield x 100

Actual yield is experimental, theoretical yield is what the balanced stoichiometry makes. Both are in MOLS.

Simply write the equation and balance it.  Then run the formula backwards.
Plug and chug, we used to call it.
You have % yield.

If this is for a lab, it should have given you the balanced stoichiometry for whatever chemicals used to make Ba3(PO4)2.  Use that.  The # in front of Ba3(PO4)2 is your theoretical yield, and if there isn't a number, use 1, which is always understood in stoichiometry to be there if they don't write a number.

Example: Table salt
2Na + Cl2 ---> 2NaCl
The theoretical yield here for table salt is 2 mols, because it's '2NaCl'.
Say your percent yield here is 88.9%
Convert % to the decimal (this takes 'x100' out of the equation)
88.9% = 0.889

Plug what you know into the formula and SOLVE FOR X.

% = actual/theoretical
0.889 = actual yield in mols/2mols
0.889(2) = actual yield in mols/2mols (2)  {see how mols cancels here?}
1.778 = actual yield in mols

Easy huh?
Now find your stoichiometry, plug and chug, and get the answer!

Hope this helps.