As  Pier Luigi Zanonato  write specific activity is A= N* decay constant

where N is the number of Cs nuclei per gm and decay constant is 0.693 over t1/2 of Cs-137

Dear Nelson. The content of the attachment could help you.

Cs Actvity, A (Bq) =Ln2 N/T1/2

Cs-137 Specific Activity, SA (B//g)=Ln2 Na /T1/2/Pa

Where Ln2 is the natural logarithm of 2,

T1/2 is the half life of Cs-137, in seconds

N is the number of Cs-137 nuclei

Na is the Avogadro´s number

Pa is the Cs-137 atomic weight

The question has been answered above.  N = N0*exp(-lambda*t) where lambda = ln(2)/H), H is the half life, N0 is the original amount, and N is the amount after time t.

I agree with a.t. farag