Chapter 8 - Graphing Quadratic Functions - 8.2 - Graphing f(x) = ax^2 + c - Exercises - Page 429: 20

$x=\pm6$

The zeros of the function are the $x$-intercepts of this function. The $x$-intercepts can be found by equating function expression to $0$ and solving for $x$: $$y=x^2-36$$ $$x^2-36=0$$ $$x^2=36$$ $$x=\pm\sqrt{36}=\pm6$$