"A que..." is a set phrase that is used to express certainty on what you're going to say.
- A que te gano la carrera.
-> I'm challenging you to compete with me and I'm sure I'll win.
- A que no sabes a quién me encontré hoy.
-> I want you to guess who I met today, but I'm pretty certain you have no idea.
- A que te sabes de memoria esta canción.
-> I'm going to play a song I'm sure you know (and even means something to you).
- A que no le dices a papá que chocaste su coche.
-> I know you're very afraid to tell dad that you crashed his car.