Answer:
321
Explanation:
21 - 15 is 6. 21 + 6 is 27. Good, now we know how much to increase the numbers each time.
15, 21, 27, 33 are 4 numbers, we need to find the 52nd.
52 - 4 is 48. there are 48 more times we need to add 6.
6 x 48 is 288. 288 + 33 = 321.
I'm not very good at math, so I'd recommend using a calculator to check your answer.
If I did something wrong, please let me know!