Chapter 12: Vectors and the Geometry of Space - Section 12.1 - Three-Dimensional Coordinate Systems - Exercises 12.1 - Page 696: 42

$5 \sqrt 2$

Formula to find the distance between two points is $|PQ|=\sqrt{(x-x_0)^2+(y-y_0)^2+(z-z_0)^2}$ Let us consider $P=(-1,1,5)$ and $Q=(2,5,0)$ Thus, $|PQ|=\sqrt{(2-(-1))^2+(5-1)^2+(0-5)^2}$ or, $|PQ|=\sqrt {9+16+25}=\sqrt{50}$ or, $|PQ|=5 \sqrt 2$