Great start! ...
Every solution begins with understanding the problem.
The next step would be to apply your formula to the user's data and determine your time constant.
I noticed you had a somewhat different formula in your original message... I don't recognize it (i.e. Its not one I've used in general electronics) but you should probably try both... When R is in Megohms and C is in Microfarads mine gives an answer in seconds... not sure about the other one.
Then, once you've got the forumla to compile, you need to display the answers... so you'll have to add some printf() with the results.
Then guess what... That's it.
