How to set a value with calculations with no setter?

I'm in need of help! So I have made methods that already run great and have been tested with unit tests! However I realized that it was needed of me to not have a setter for a get. This pretty much gave me errors and now I'm stuck.

This photo shows a unit test I made to test out if the debit was valid.

This was my getter and setter. And since taking out the setter I have gotten errors. I know that I have to do something with the return value but am unsure what I should do. Two methods are connected to the balance.

I am unsure on how i'd be able to make the calculations without having a setter. Please help!