Apologies, this was just the manipulation of the RHS. I see what they've done now - multiplied through by 1/(D(x)p(x)) to give RHS of 1/(D(x)p(x)) d/dx (D(x)p(x)) then applied the chain rule to give RHS = ln d/dx (D(x)p(x)). Then this equals v(x)p(x)/D(x)p(x) = v(x)/D(x). Thanks