EDTA/ Hardness of Water PLease i need all the help i can get

[dpascali]

Part1: A 0.5215-g sample of CaCO3 is dissolved in 12M HCl and the resulting solution is diluted to 250.0mL in a volumetric flask.
There is a second part to this problem that has also taken me many hours to figure out how to set up.
the set up is:
25.00 mL aliquots of the solution from problem 1 are titrated with EDTA to the Eriochrome Black T end point. A blank containing a small measured amount of Mg 2+ requires 2.60 mL of the EDTA to reach the end point. An alquot to which the same amount of Mg 2+ is added requires 28.55mL of the EDTA to reach the end point.
the questions are:
a.) how many mL of EDTA are needed to titrate the Ca 2+ ion in the aliquot?
would you just add the 25.00mL and 28.55mL then subtract the 2.60 mL from that answer to get the total?
b.) how many moles of EDTA are there in the volume obtained from Part a?
PLease i need to get all the help i can get, i've been working at this for quite a few hours.