Problem $3(25$ points$)$:

$($a$)$ If the magnitude of the force $F$ is $300$ N $,$ determine the magnitude of the moment produced by the force about the $x$ axis. $(15\%)$

$($b$)$ The friction at sleeve A can provide a maximum resisting moment of $100N*m$ about the $x$ axis. Determine the largest magnitude of the force $F$ that can be applied to the bracket so that the bracket will not turn. $(10\%)$