assuming that is your actual function, you are saying that the function returns a float, is named slope, and takes a float as a parameter, however you are not saying what you will be calling the float passed to it, the way you have written it is how you would write a prototype, try writing it more like this: float slope (float var) and replace var with a meaningful name that describes what the value you are passing to it is. use this variable name everywhere in your function that you need to use the float that was passed
The error message you are receiving could mean that you are not giving a name to the argument that you can refer to. You should not use the same name of the variables as the type for this won't work with a typedef. Make sure also that the code that you use for the next item shows that you expect an int.