You could use the exponential function in a general form and start fitting it to the two conditions you have, however there is a slightly more advanced way. Write out the two conditions as:
[tex]f(0)=20[/tex]
[tex]f(7x)=2f(x)[/tex]
Take the derivative of the second one and use in it the original again. You will end up with a first order differential equation giving as solution the exponential function. Using the first equation as boundary gives then the function you are looking for.