Chapter 8 - Graphing Quadratic Functions - 8.2 - Graphing f(x) = ax^2 + c - Exercises - Page 429: 21

$x=\pm5$

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 $x$: $$f(x)=-x^2+25$$ $$-x^2+25=0$$ $$-x^2=-25$$ $$x^2=25$$ $$x=\pm\sqrt{25}=\pm5$$