If f(x) [f of x]is a twice differentiable function where for all real numbers f'(x) [f prime of x]is less than 0 and f''(x) [f double prime of x] is less than 0, with f(4) = 12 [f of four equals twelve] and f(5) = 9 [f of five equals nine], which of the following is a possible value for f(6) [f of six]?