titrating calcium ions

The concentration of calcium ions in hard water can be determined by titration with a reagent X which forms a complex with Ca
2+
(aq), giving a change of colour. Three moles of X combine with one mole of Ca
2+
(aq).
A 25.0cm
3
sample of hard water reacted with 24.0cm
3
of 1.00×10
−4
mol dm
−3
X(aq).
Determine the concentration, in mol dm
−3
, of calcium ions in the hard water