Predictions of Malthus' model

This applet shows predictions of the Malthus' model, , corresponding to different values of and .
Observe how the number of individuals changes with time for different values ​​of and of . Find the function which allows directly calculate the value of from , , and , , without calculating the intermediate values ​​from to , and enter it in the input box. Remember that to let GeoGebra consider it as a function you have to use instead ot .