What is the molarity of a 15% solution of Ni3Fe(CN)6?

I believe nickel ferricyanide has the formula:

Ni3(Fe(CN)6)2
which makes quite a difference to the molarity. Please check the question.

If the 15% solution is mass/volume, then a litre contains 150g of the compound.

The RMM (relative molecular mass) can be obtained by summing the relative atomic mass of the individual elements.
Divide 150 g/l by the RMM gives the molarity in M.