To answer this one, recall that the "half life" is the amout of time that it takes for the mass to be reduced to 1/2. So, in 1600 years, the mass will be 1 gram, in 3200 years, 0.5 grams. Every 1600 years, the mass will be cut in half. Keep doing this and you'll see that after 9600 years (which is 6*1600), the mass will be (1/2)^6 * 2 grams. For part a, just start with the decay exponention function for mass (e. g. N(t)=No*e^(-lambda*t)) and plug in the values that were given for the half life (N/No= 1/2) and solve for lamda. When you solve for lambda, you will have the formula.