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.

$$ \frac{T(2N)}{T(N)}=2^b$$ .

$$ T(N_0)=T $$.

I am not sure how to proceed after that.