mtw Definition and 1 Threads

I need help with exercise 21.26 in MTW. The question goes like this: For a thin shell of dust surrounded by vacuum ( ##[T^{in}]=0## , ##\mathbf{t}=0## ), derive the following equations$$\frac{d\sigma}{d\tau}=-\sigma^b{}_{|b}\;\; ,\tag{21.175a}$$$$\mathbf{a}^+ +\mathbf{a}^- =0 \;\...