We are doing a grams to moles to atoms conversion.
You will need Avogadro's Constant and the amount of grams per mole of Calcium from whichever periodic table you happen to have.
I used Ptable.
You can always double check to make the proper units are left in numerator of the last conversion factor.
For proper significant figures, if you were given the value (80g) for Calcium, there is only one significant figure in that number.
Thus, your answer would only be good to one significant figure: