You've done so much prep, I'm sure you'll do great on the test! Go for it!
After all your studying, you're sure to do well! Try hard!
prep = preparation (準備、つまり、練習、勉強)
「あなたなら」を直訳したら、「in your case」「because it's you」になりますが、でも自然に、その意味が「You've done so much prep」「After all your studying」の中に含んでいると思います。