Welcome to PF (physicsforums)! If you haven't had a chance to look around, you will find a lot of fascinating and useful discussions here. If you have something puzzling you, there are quite a few knowledgeable people here, who can point you in a successful direction.
Are you building this circuit for a home project? Is this a question from school?
If this is a homework question, there is a different forum section to post your question. Don't worry, we can easily move this to the proper place if it is a homework question.
It is a good idea to show how you arrive at a number, even if you don't know how to encode it in a pretty format.
for example the area of a circle may be typed A= pi *r^2 .. When you become more familiar with formula coding (we can show you where to see examples), you can make it look like this: A = \pi r^2
If you were solving for "r", you could type r=squareroot[A/pi] or sqrt[A/pi]. Or you could even point to a webpage that shows the formula you are attempting to use (if there are more than one on that page, explain which one). e.g. for Area of circle, i used the formula for area on this page -->
http://www.worsleyschool.net/science/files/circle/area.html
What formula did you use to find C ?