Chapter 8 - Graphing Quadratic Functions - 8.2 - Graphing f(x) = ax^2 + c - Exercises - Page 429: 24

$x=\pm3$

The zeros of the function are the $x$-intercepts of this function. The $x$-intercepts can be found by equating the function expression to $0$ and solving for $x$: $$f(x)=3x^2-27$$ $$3x^2-27=0$$ $$3x^2=27$$ $$x^2=9$$ $$x=\pm\sqrt{9}=\pm3$$