first of all, you need the charge and formula for each ion: the nickel(II) ion and the manganate(VII) ion.
Spoiler
since nickel(II) manganate(VII) is an ionic compound, what must the overall charge of the compound be?
Spoiler
so, using all this information you now have, can you work out the ratio of nickel(II) ions to manganate(VII) ions in the compound, so that you get the overall charge?
What is the ionic charge and formulae for nickel(II) manganate(VII)?
It is an ionic compound, it means that the opposite charges (positive and negative) have to be zero overall. You need to know each number of ions in the compound and their oxidation numbers to calculate.
Example:
K2Cr2O7 (potassium dichromate)
Each potassium has the oxidation number +1, there are two, so 2*(+1) = +2 Each chrome has the oxidation number +6, there are two, so 2*(+6) = +12 Each oxygen has the oxidation number -2, there are seven, so 7*(-2) = -14
The sum of these numbers results in the overall charge zero: +2 +12 -14 = 0
Zero means that the charges are balanced to make an ionic compound.
first of all, you need the charge and formula for each ion: The nickel(ii) ion and the manganate(vii) ion.
Spoiler
since nickel(ii) manganate(vii) is an ionic compound, what must the overall charge of the compound be?
Spoiler
so, using all this information you now have, can you work out the ratio of nickel(ii) ions to manganate(vii) ions in the compound, so that you get the overall charge?