To answer for the first question, divide 828 days with the half-life, 138 days. There are 6 half-lives of polonium-210 in 828 days.
For the second question, raise 0.5 to number of half-lives which was calculated in a, 6. Then, multiply with the initial amount, 61 g. Thus, after 828 days there are only about 0.9531 g of polonium-210 left.
