Question

a solution is prepared by dissolving 15.0 grams of ni(no3)2 in enough water to make 250 ml of solution what is the molarity of this solution

Answer 1

I assume you mean Ni(NO3)2. In chemistry you need to find the caps key on your keyboard and use it. CO, Co, and co are not the same.
mols Ni(NO3)2 = grams/molar mass = ?
Then M = mols Ni(NO3)2/L solution = ?

Answer 2

0.0229 M