Apologies for this simple question. I found it in the book Algorithms by Sedgewick and Wayne.
Give a formula to predict the running time of a program for a problem of size N when doubling experiments have shown that the doubling factor is $2^b$ and the running time for problems of size N0 is T.
$$ \frac{T(2N)}{T(N)}=2^b$$ .$$ \frac{T(2N)}{T(N)}=2^b.$$
$$ T(N_0)=T $$.$$ T(N_0)=T. $$
I am not sure how to proceed after that.