A local gym sells monthly memberships. The function f(x) = −x2 + 18x − 72 models the daily profit in dollars, where x is the number of memberships sold. Determine the zeros, and explain what these values mean in the context of the problem.

x = 9, x = 18; The zeros represent the number of monthly memberships that produces a maximum profit.
x = 9, x = 18; The zeros represent the number of monthly memberships where no profit is made.
x = 6, x = 12; The zeros represent the number of monthly memberships that produces a maximum profit.
x = 6, x = 12; The zeros represent the number of monthly memberships where no profit is made.