You have to find out the number of moles of the substance.
I will use rounded numbers for the atomic weights because every periodic table is different.
2Ca = 2 * 40 = 80
1 C = 1 * 12 = 12
3O = 3 * 16 = 48
Atomic Mass = 140
moles = given mass / molar mass
moles = 635.94 / 140
moles = 4.54
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Now you need to find the concentration
C = moles / Volume in liters.
C = 4.54/ 2.0 L
C = 2.27 moles / L
Technically since the volume is only 2 significant digits, your answer should be 2.3 moles / L
If you do not have this as a choice, what are your choices.