rf is the risk free rate. The YTM on 10 year US Treasury securities is a good approximation.
You may assume a corporate tax rate of 40%.
The synopsis of Kellogg Company should include
contains the following elements:
1. The calculated WACC.
2. How data was used to calculate WACC. This would be the formula and the formula with your values substituted.
3. Sources
4. A discussion of how much confidence in the answer. What were the limiting assumptions that you made, if any.