Consider a circle with the area A circumference of C radius of r and diameter d write d as a function of r and show work
There is a lot of information here you do not need. One of the keys to solving word problems is to eliminate the stuff that is there to confuse you.
The main thing to pull out is that it is a circle. So, think about your properties of circles. How are radius and diameter related? That is all you have to answer. A function is just a relationship statement.
How is the diameter related to radius?