Chapter 8 - Graphing Quadratic Functions - 8.1 - Graphing f(x) = ax^2 - Exercises - Page 424: 22

$f$ is decreasing when $x<0$ $g$ is decreasing when $x>0$

When $a>0$ the function $f(x)=ax^2$ is decreasing when $x<0$ as the parabola has a minimum in the vertex. When $a<0$ the function $g(x)=ax^2$ is decreasing when $x>0$ as the parabola has a maximum in the vertex. So the $f$ function is decreasing when $x<0$ and the $g$ function is decreasing when $x>0$.