When you set up this equation you want the number of grams you start with, 2616, and the factor by which it is decreasing, which is half, or 1/2. Because the half life is 11 days, but you want to find the amount left over after 22 days, you can make 22/11 as the exponent on the 1/2. The equation should look like this : 2616(1/2)^22/11. With these kinds of equations you always want to multiply the starting value by the factor of increase/decrease, and raise that factor to the time eclipsed divided by the set time(like the 11 day half life).
