Please use code tags (first button in the Format box next to where you enter the text). It will make your posting more readable.
I noticed that your function definitions do not match function implementations. For example you define float getPayment(); but you implement float getPayment(float balance,float APRCharge). If your function takes two float arguments, it has to be described as such in the definition float getPayment(float,float);
Start from there, and post any error messages that you get
I don't see how this code can even compile. Inside your getAPRCharge function, you're attempting to use something called balance, but I don't see a definition for that.
In your original unformatted post, getAPRCharge took an argument called balance. Did you remove that by mistake?
EDIT: Also, your code would be much easier for both you and us to understand, if you used a sensible indentation style.
Your definition should be float getAPRCharge(float);, with the implementation as in your first posting.
When you say it's not compiling, please post the error message. In fact, I am almost sure that your error message tells you what is wrong in your code.