let's call our number hmmm Z, now, let's reduce it by 17%, so 100% - 17% = 83%, so the new size for Z is 83% off the original, hmm how much is that?

now, let's increase it by 12%, so the new size will be 100% + 12% = 112%, so 112% of 0.83Z, let's check how much is that

now, let's convert that to a percent format by simply multiplying it by 100, so that'd be 100 * 0.9296Z = 92.96% of Z. Well, hell 92.96% is less than 100%, so is really 100% - 92.96% = 7.04% less.
So, instead of all that rigamarole, we could have just reduced Z by 7.04% in one fell swoop and obtain the same thing.