The first picture, at the first question mark: F(x) is regarded as the unknown t of G(x), t = F (x) = 2x- 1, and G(T)=G(2x- 1).

The second question mark: Like the first explanation, G(x) is considered as a piecewise function, so it needs to be discussed separately, that is, t is greater than or equal to zero and less than zero.

The third question mark: because the unknown number is less than zero, the corresponding function values are all-1, so the result remains unchanged.