0.000048 is how much your chance of getting a brawler increases every 30 boxes, so we multiply it by b/30 to account for the amount your chance will increase. For example, if you open 30 boxes, 0.000048(30/30)=0.000048 which is the amount your drop rate has increased! We then subtract this number from 1, alongside a (which accounts for your original drop rate) to receive the chance that you will NOT get a brawler from a single box. After this, we raise that number to the power of b, which in this case is 30, in order to find out the total chance of NOT getting a legendary in B boxes. Finally, we have to subtract all of this by 1 in order to find the chance that we WILL receive a brawler!