You can put this solution on YOUR website! y is the terms, x is the index of the term, starting at 1. ----the model to try.. ---- should be a linear equation.
You can plot points, (x, ln(y)), which will probably be a line. Get k from the graph, and you already know the value of a. If not, you will find it on the graph as the vertical axis intercept. See for ln(a), the vertical intercept.