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You can solve your problem using the equation where N(t) is the number of atoms at a given time t, N_0 is the number of atoms at t = 0, and lambda is the decay constant.
The activity (A) can be written as so we can multiply both factors for lambda and have the equation that will solve our problem:
since we know A(t), A_0 and t from the data of our problem we can find lambda.
Since the half-life of our isotope is t_(1/2) = 2.32 hours.
I really hoped this helped you! Let us know if you need any more clarification on the matter of radioactive decay!