Answer
The magnitude of the resultant force is 94 lb
Work Step by Step
Let $a = 40~lb$
Let $b = 60~lb$
Let angle $\theta = 40^{\circ}$
We can use the parallelogram rule to find $c$, the magnitude of the resultant force:
$c = \sqrt{a^2+b^2+2ab~cos~\theta}$
$c = \sqrt{(40~lb)^2+(60~lb)^2+(2)(40~lb)(60~lb)~cos~40^{\circ}}$
$c = \sqrt{8877.01~lb^2}$
$c = 94~lb$
The magnitude of the resultant force is 94 lb