Question

Predict the formula for a compound made from X3 and Y3–.

Answer 1

Well, a compound has a total charge of 0. So, it's electrically neutral. Since the X is 3+ and the Y is 3- they add to 0. Meaning no subscripts are necessary. Why don't you try a different combo?

Like: 

A^3 and B^1-, to get a 3- charge you need 3xB^1- so the formula is AB3

Does this help?  

Answer 2

Answer: The formula for a compound made from $X^{3+}$ and $Y^{3-}$ is $XY$.

Explanation:

For formation of a neutral ionic compound, the charges on cation and anion must be balanced. The cation is formed by loss of electrons by metals and anions are formed by gain of electrons by non metals.  

Here cation is having an oxidation state of +3 written as $X^{3+}$ and anion is having an oxidation state of -3 written as $Y^{3-}$. Thus they combine and their oxidation states are exchanged and written in simplest whole number ratios to give neutral $XY$.

Thus formula for a compound made from $X^{3+}$ and $Y^{3-}$ is $XY$.