Chapter 11 - Section 11.1 - Three-Dimensional Coordinate Systems - Exercises - Page 600: 43

$7$

Let $A=(1,4,5)$ and $B=(4,-2,7)$ be the two given points. The distance between two points can be calculated as: $|AB|=\sqrt{(x-x_0)^2+(y-y_0)^2+(z-z_0)^2}$ Thus, we have $|AB|=\sqrt{(4-1)^2+(-2-4)^2+(7-5)^2}=\sqrt {9+36+4}=\sqrt{49}=7$