Player one (P1) picks a number between 100 and 0 inclusive. Let's call this number X. Whatever the X P1 has picked, it is subtracted from 100 and now is in front of P1. So P1 has 100 - X in front of him. This X amount is then delivered to P2. Then P2 picks a Y amount between X and 0 inclusive. That Y amount is deducted from P2, multiplied by 3 and added to whatever was in front of P1.
So at the end, P1 ends up with 100-X+3*Y and P2 ends up with X-Y. What is the Nash Equilibrium for this game?
No matter what strategy player one picks (no matter which $x$ is chosen) it is always in the best interest of player two to pick $y=0$. Given that player one knows that player two will always pick $y=0$, the best strategy for player one is to pick $x=0$.
The notion of equilibrium you want to use in this kind of extended form game is called subgame perfect equilibrium, and it is most easily solved via backwards induction. The linked article covers some small examples of this.