The half-life of a particular element is 280 days. what is its decay constant?

The formula for decay is
                                      N' = Ne^(-kt)
with N as the initial amount, k as the decay constant, and N' as the final amount. For decay, 280 days is the time it takes for the initial amount N to decay to 1/2N (half its original mass).
                                 1/2A = Ae^(-280k)
                                 1/2    =   e^(-280k)
Solving for k, we will yield
                                 ln(1/2) = -280k
                                          k = ln(2) / 280 day^-1
                                          k ≈ 2.5 × 10^-3 day^-1

True, the standard unit for volume is liters which can be represented by either a lower case "l" or a capital "l"