For (ii), you need to know the definition of an element. If I recall correctly, it is a pure substance that cannot be split into two or more simpler substances via chemical processes or electricity.

Sodium Nitrate has the chemical formula NaNO3 which means one molecule can be split up into 1 atom of sodium, 1 atom of nitrogen and 3 atoms of oxygen.

So your answer for (ii) should reflect the following points:

A sodium nitrate molecule is made up of 3 different elements (sodium, nitrogen and oxygen).

An element is a pure substance that cannot be split up into two or more simpler substances by chemical or electrical means, but sodium nitrate can.

^ Paraphrase these two points and add in your conclusion. :)

For part (i), I am going to take a guess ...

The test for the nitrate anion involves warming a nitrate solution with aluminium and see if ammonia gas is produced.

So following this logic -
I would hydrate a known amount of sodium nitrate and heat it with aluminium then measure the ammonia gas produced.

*Hydrate means to add water to the substance.