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Help for finding the equation

Hello

i have a diagram which is consisted form two Cosine like graphs(as shown in the picture) but with different amplitudes,any ideas what could be an equation that rules this graph and is compatible with it?

i thought maybe y=a*(e^(bx))*Cos(cx) maybe a good equation but there is a problem,how i don't know how to eliminate two of a,b and c two find the third one,(cause the equation is not a simple linear equation),

so any ideas that what can be another equation that can be compatible with this graph?or any ideas that how i can obtain a,b and c with having 3 points of the graph?

thanks