Which of the following functions has a maximum at x=3? select all that apply. a. f(x)= (x-3)^2 b. f(x)= (x-2)(4-x) c. f(x)= -x^2+6x-5 d. f(x)=1-(x-3)^2 e. f(x)= (x+3)^2-9 f. f(x)= x^2-4x+3