Multiple regression then Sobel for mediation


I am doing a mediation analysis. I first ran a multiple regression for IV (2 variables) to mediator. I want to do a Sobel test and use the unstandardized coefficient of the 2 IVs combined. However, I'm not sure where do find that number. Is it the "constant"

Is it the R?
Is it neither??

I hope this question was clear. Thanks!