You need to pass "monthlyIncome" to "computeTaxes(...)". Also you should do something with the value returned from "computeTaxes(...)" right now it's not getting saved anywhere.
K. I edited it a little. I guess what I'm having trouble with is where do I pass "monthlyIncome" to "computeTaxes()". Do I have to put it in the function I called in main or in the "computeTaxes()" function itself?
You have a scope issue with "budgetTaxes", it is a variable inside of your "computeTaxes(...)" function but Main() has never heard of it. You need to declare a variable for your function to return to and use it, have you read that link I gave you? It'll address all of this much better then I can.
the program is fine except fora few thing. first off the function is computeTax not computeTaxes so change the call in your main. secondly, you havent stored budgetTaxes anywhere. just returning it isnt good enough, because the variable goes out of scope once the function is ended. so instead do this in main: