The steps in writing f(x)=18x+3x2 in vertex form are shown, but a value is missing in the last step. Write the function in standard form. Factor a out of the first two terms. f(x)=3(x2+6x) Form a perfect square trinomial. (six-halves) squared f(x)=3(x2+6x+9)−3(9) Write the trinomial as a binomial squared. f(x)=3(x+___)2−27 What is the missing value in the last step? 3 6 9 18