You don't need to memorize them, there's a simple way to do it. You already know that -log 1 x 10^-1 = 1, -log 1 x 10^-2 = 2, -log 1 x 10^-3 = 3, etc.. So using this logic, you can estimate what -log 3 x 10^-7 is by saying that -log 3 x 10^-7 is in between -log 1 x 10^-7 and -log 10 x 10^-7 (because 3 is in between 1 and 10). Then, convert -log 10 x 10^-7 into -log 1 x 10^-6 (if you make the 10 smaller you have to make the power bigger). So now we know that -log 3 x 10^-7 is in between -log 1 x 10^-7 and -log 1 x 10^-6. We know already know what each of the last two values are, they're 7 and 6. Therefore our answer must be between 7 and 6, and since 3 is much closer to 1 than it is to 10 (the 10 came from the -log 10 x 10^-7 that we said was -log 1 x 10^-6), we can estimate that the answer must be much closer to 7 than it will be to 6, maybe like ~6.7. And you'd most likely only have one answer choice that's very close to your estimate, in this case it was 6.5. It seems long but you'll get faster at it with practice, I'm not sure how often this would show up on the dat though.