Probably quite straight forward...

I haven't done mv calculus in a while ... I'm stuck trying to understand a line of a solution I've been given. For this question T is a constant and x and t are variables.

If

- I'm OK with this

- And this

using

I can see how the last equality holds given the "using...", but I have no idea where that came from. Any help?

Possibly there is some bad notation going on with the first and last statements - it should be f'(x*) = g(x*) and g'(x*) = -x*g(x*) or something, maybe...