How are we able to substitute the transport function into our rendering equation to express it in the form as it is at the bottom of the slide

@arjun If you look carefully, we see that the $L_i(p, \omega_i)$ term in the first equation of the slide is replaced by $L_0(tr(p, \omega_i), - \omega_i)$ in the boxed equation. This is exactly the equation we derived using the transport function. $tr()$ here is the transport function.