$3.)$ The structure consists of two bars, $AB$ and $AC,$ joined together by a pin at $A.$ An external force of $400lb$ is applied to the pin at $A$ as shown. For the structure to remain in equilibrium, the two bars $(AB$ and $AC)$ must work together to exert a total force of $400lb$ in the opposite direction. Bar $AB$ exerts a force in a direction parallel to $AB$ and bar $AC$ exerts a force parallel to $AC.$

Using either the triangle rule or parallelogram law, determine the force exerted by bar $AB$ and the angle $$ if the resultant of the two forces is $400lb$ going to the right. Take the force in $AC$ to be $500lb$ and the angle $=40.$

For clarification: You are looking for a force acting in the direction of $AB$ that adds together with a force acting in the direction of $AC(500lb)$ to give a resultant of $400lb$ force acting horizontally to the right.

$[$Answer: $=53\mathrm{.}5,F_{AB}=621lb]$