A to power -1 + b to power-1) ÷ (ab) to power -1​

You can’t have negative exponents you you just flip the fraction

So don't take my word i'm not exactly a mathematical person but i would assume a. because while selling the soap every month you increase in sells so you would by 11 percent. the you would get the factor 0.89 which if i had to guess would be the monthly profit over 11 probably wrong tho