What is the correct ionic formula when mg2 + and p3 - react?

Any compound should have a neutral charge. So the compound you make should have a net charge of zero.

So you have:

Mg which is 2+

And P which is 3-

You want to balance out both their charges. If you simply made MgP it would have a negative charge (because 2-3 = - 1.)

Now to balance this, you have to find the LCM: In this case it is 6. (2x3)

Mg: Mg x 3

P: P * 2

You'll finally get Mg3P2.