WACC: pension assets, operating assets beta

I thought I had this section pegged then missed a question on the last of the Schweser exams. When you are not given a cost of debt, how do you calculate the WACC? I believe it has something to do with the operating asset beta but am not sure. And then how would you go about calculating the operating asset beta? Thank you,

Use CAPM approach.

Are you saying use the operating asset beta instead of total asset beta in the CAPM formula and that will provide me with the WACC? What is the formula to obtain the operating asset beta?