Answer
The magnitude of the resultant force is 1286.0 lb
Work Step by Step
Let $a = 1500~lb$
Let $b = 2000~lb$
Let angle $\theta = 140^{\circ}$
We can use the parallelogram rule to find $c$, the magnitude of the resultant force:
$c = \sqrt{a^2+b^2+2ab~cos~\theta}$
$c = \sqrt{(1500~lb)^2+(2000~lb)^2+(2)(1500~lb)(2000~lb)~cos~140^{\circ}}$
$c = \sqrt{1653733.34~lb^2}$
$c = 1286.0~lb$
The magnitude of the resultant force is 1286.0 lb