To determine the formula of magnesium and carbonate using the criss cross method, we first write the symbols of the elements and their charges:

Magnesium: Mg +2

Carbonate: CO3 -2

Next, we criss cross the charges to obtain the subscripts for the formula:

Mg2(CO3)1

Finally, we simplify the formula by removing the subscript 1 from the carbonate group:

Mg2CO3

Therefore, the formula for magnesium carbonate using the criss cross method is Mg2CO3.