Chapter 8 - Section 8.1 - The Square Root Property and Completing the Square - Exercise Set - Page 595: 85

$28.28$ or $28.3 $ (approximately)

Apply Pythagorean theorem. $b^2=c^2-a^2$ Plug $c= 30 ft$ and $a= 10 ft$. We have $b^2=900-100$ or, $b^2=800$ or, $b=\sqrt {800}$ or, $b=\sqrt {(40)(2)}$ or, $b=20\sqrt 2=28.28$ or $b=28.28$ or $28.3 $ (approximately)