What is the molarity (to three decimal places) of the resulting solution when 46.870 grams of Mn(ClO4)2·6H2O are added to 300.0 mL of water?

Can't do it: Molarity is moles of solute per liter of SOLUTION. You do not know the final volume of the solution; the 300 ml of water must surely increase in volume when the MgClO4 is dissolved in it. Do you have the density ol the resulting solution? Did you phrase your question exactly as written? My thought is that you are supposed to determine the weight of anhydrous MgClO4 dissolved, (The water of hydration would not contrilbute to the Molarity so you need to determine the % MgClO4 in the sample).

Yes. I copied and pasted it right from my webassign. Thanks anyway though.

If you have 300ml of SOLUTION you can do it, but not as written.