Alright so first you want to find out how you take 30% off of $30. The first step is to ask yourself, what's 30% of $30? All you have to do is make the percentage into a decimal like you did and multiply .30 times 30. That gets you 9. So 9 is 30% of $30, but since it said the original 30 was marked down by that much, it means subtract. 30-9=21.
For the second part, you pretty much do the same thing but using the 21 you just got and taking away 20% this time. Same process so .20 times 21=4.2, and take the 4.2 away from 21 to get 21-4.2=16.8.
Convert that to dollars to see that the final price is $16.80.
Just a tip, a lot of people will forget the subtraction part after they find the percentage part. Most of these type questions are two steps like that so don't forget to subtract when needed. Lmk if you still have questions :)