F(t)=-13+4i in quadratic formula

Hello people, i have a math question that i need with, me, i will give the correct one brainiest, and for the other correct people, a and 5 stars.

Li buys supplies so he can make and sell key chains. he begins to make a profit only after he sells 4 key chains. what point on the graph makes the most sense in this situation?

F(n + 1) = f(n) – 8. if f(1) = 100, what is f(6)?