Chapter 12 - Vectors and the Geometry of Space - 12.1 Exercises - Page 815: 35

$0\lt x \lt 5$

The equation of the $yz$-plane is $x=0$. So if a point lies between the planes $x=0$ and $x=5$, its $x$-coordinate must be between $0$ and $5$. Hence our region is the set of all points $(x,y,z)$ with $0\lt x \lt 5$.