Appendix D - Graphing Techniques - Exercises - Page 448: 74

Another function value is ƒ(-3) = 6.

For all $x$, ƒ$(-x)$ = ƒ$(x)$ shall mean that the graph of an equation is symmetric with respect to the $y$-axis since the replacement of $x$ with $-x$ results in the original equation again. So, for a function ƒ, ƒ(3) = 6, another function value is ƒ(-3) = 6 then.