I'd recommend using a website to make the calculation, but in case breaking it apart is helpful...

400/135 = 2.963 (the total growth)

Since it took 25 years, you want to know what number to the 25th power results in 2.963.

x ^ 25 = 2.963

to solve that exponential means taking the log of both sides. I'm using "log base 10" for this:

log (x ^ 25) = 25 * log x ; log (2.963) = .4717 (aka, 10 ^ 0.4717 = 2.963)

25 * log x = 0.4717

divide both sides by the years:

log x = 0.4717 / 25 = 0.01887

To get rid of the (log x), we take 10 raised to the power of both sides of the equation:

10 ^ (log x) = x ; 10 ^ 0.01887 = 1.044

x = 1.044

Given a growth of 2.983 times over 25 years, that is like an average growth of 4.4% per year.