Hi @nathaliavs,
This question is asking you to recall the 3 db rule of sound intensity, which is this: If you increase the intensity of a sound by 3db it will double the audible noise, and if you reduce it by 3db it will halve the amount of audible noise.
Recall from your environmental controls classes that sound intensity is not linear; it is logarithmic. The formula to calculate a difference in two sound intensities is this: 10 Log 10 (S1 / S2), where S1 and S2 are the intensities of your two sounds.
I found a nice, fairly simple explanation for you on Encyclopedia Brittanica: https://www.britannica.com/science/decibel
More generally I would look at the NCARB ARE 5.0 Handbook for any formulas that you will be expected to know. This particular formula is not listed by NCARB but this 3db rule is an important principle to be familiar with.
Hope it helps,