You could crack a code, but that doesn't mean it will be easy or fast. "I'm going to ace the exam" is appropriate to say what you mean, but you wouldn't ace the car repair, I don't think. You could also say, this exam will be a piece of cake. Getting my car repaired will be a piece of cake. "I'm a whizz at math" means math is really super easy and fast for me. You might say "I'll whizz right through the exam." Or "It will be a breeze." I'll breeze right through the exam. Getting my car repaired was a breeze.