Answer: 504:420:252
Explanation:
Here is what I did, I'm not sure if it is what you're teacher wants though.
1. I took 420 and divided it by 3. It equals 84. This tells me how many times louder it was than the original. I know that since it is a ratio, as one of the numbers in the set go up, all of the numbers go up. For example if the headphones increase by 1 decibel, it would be 7:6:4. This tells me how many times louder it is. I hope that makes sense...
2. Since it's 84 times louder, we can just take 84 times the other numbers.
84*6=504
84*3 = 252
3. Final answer would be 504:420:252