assuming you know how to derive : t(1/2)= ln(2)/lambda , from the N(t)=N0 * exp(-lambda*t) and N(t(1/2))= half of N0
and the rate of dacay(disintegrations per second) is proportional to the number of nuclei, by computing dN/dt
i.e. Activity= lambda*N
so you can get lambda from the 1st eqn...