Chapter 12 - Vectors and the Geometry of Space - 12.4 Exercises - Page 839: 40

$196.7$ ft-lb

$| \tau|=|r \times f|= |r||f|sin \theta$ $|r|=r=\sqrt {4^2+4^2}=4 \sqrt 2$ , $|f|=36 lb$, $\theta=135 ^ \circ-30 ^\circ=105^\circ$ $| \tau|=|r \times f|= |r||f|sin \theta$ $=4 \sqrt2 \cdot 36 \cdot sin (105 ^ \circ)$ $ \tau =196.7$ ft-lb