Please can someone help me with a formula?

I don't even know the maths I need to work this out...

But I have a cost per KM at a start point, and then I know that the cost increases x% per month, and I have an average KM per month... How do I work out what the cost would be at specific points during a term of x months?

I think this is more of a maths question than an Excel question, but I need to use the formula of this to get the answers for several different instances of this question.

Any help would be huuuuugely appreciated!