What is the vertex of the graph of the quadratic function

Hmmm i had this before , give me a sec

Here is my answer, i'm sorry if you didn't understand my explanation

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