Chapter 12 - Vectors and the Geometry of Space - 12.5 Exercises - Page 849: 70

$\frac{5\sqrt {5}}{3}$

Distance formula is given as: $d=\frac{|a \times b|}{|a|}$ Cross product: $a \times b=\lt -5,0,10\gt$ $|a \times b|=\sqrt {(-5)^2+0^2+(10)^2}=5\sqrt {5}$ $a=\sqrt {(2)^2+(-2)^2+(1)^2}=\sqrt {9}=3$ Thus, $d=\frac{5\sqrt {5}}{3}$