An equivalent is described as an amount of substance that in an acid base reaction reacts with or in a redox reaction supplies one mole of H+ ions or reacts with one mole of electrons.
For Ca2+, as 1 mole of Ca2+ reacts with 2 moles of electrons in a redox reaction, one equivalent of Ca2+ is two moles of Ca2+.
The solution in the problem has a concentration of 2.5 mEq/L of Ca2+. As 1mEq of Ca2+ is 2 moles of Ca2+, 2.5 mEq of Ca2+ is equivalent to 2.5*2 = 5 milimole of Ca2+ per liter.
The concentration of the solution that is 2.5 mEq/L in terms of mEq/L is 5 mmole/L.