The stuff inside the parentheses are the function's parameters. They are the variables that you give the function in order to enable the function to do its job.
Your "findcircumference" function doesn't need to know what circumference is; that's what it's supposed to tell you. But it does need to know what radius is in order to calculate the circumference for you.
The "int" in front of "findcircumference" is the return type of the function. Since it is an int, then the function returns an integer. But you need circumference to be a double, so you should change that int to double.