Hi Daisy,
Before we can start calculating we need to identify which of our solutions contains Fe2+ ions, since the question specified that ionization. That would be the FeCl2 since the other one has 3 NO3 ions, which is a 1- charge, whereas the former solution has only 2 Cl ions.
Now that we know what the focus is, lets start calculating, First we need to find the moles of Fe2+:
56.1 mL/1000 = 0.0561 L
Molarity = moles/L
0.51 = n/(0.0561)
n = 0.028611 moles of Fe2+
Because there is only one Fe2+ ion, we don't have to do any multiplication.
Now we need to find the total new volume when combining the 2 solutions together:
V = 0.0561 + 0.040 = 0.0961 L
We will use this now to find the new molarity concentration:
Molarity = moles/L
Molarity = 0.028611/0.0961
Molarity = 0.29 M
Please let me know if that made sense and if you have any questions!