#### Answer

The magnitude of the resultant force is 29 lb

#### Work Step by Step

Let $a = 18~lb$
Let $b = 12~lb$
Let angle $\theta$ be the angle between these two forces. Then $\theta = 10^{\circ}+15^{\circ}= 25^{\circ}$
We can use the parallelogram rule to find $c$, the magnitude of the resultant force:
$c = \sqrt{a^2+b^2+2ab~cos~\theta}$
$c = \sqrt{(18~lb)^2+(12~lb)^2+(2)(18~lb)(12~lb)~cos~25^{\circ}}$
$c = \sqrt{859.5~lb^2}$
$c = 29~lb$
The magnitude of the resultant force is 29 lb