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
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.