Question 15Let $f$ be a function for which $f\!\left(\dfrac{x}{3}\right) = x^2 + x + 1.$ Find the sum of all values of $z$ for which $f(3z) = 7.$$-\dfrac{1}{3}$$-\dfrac{1}{9}$$0$$\dfrac{5}{9}$$\dfrac{5}{3}$