SOLUTION: Two tow trucks try to pull a car out of a ditch. One applies a force of 1500 lbs, the other 2000lbs. The resultant force is 3000lbs. What is the angle between them?

The other tutor's solution is incorrect!

Here are the two forces.  We want to find 



To draw the resultant, we first draw 2 lines, one 
through each arrowhead parallel to the other vector.
This completes a parallelogram.
I'll draw them in green below.  Notice that the angle 
labeled  is the supplement of angle 

 

Then the resultant is the vector whose tail coincides
with the two vectors, and whose arrow tip is at the
opposite vertex of the parallelogram.  I'll draw the
resultant in red.  We are told that this resultant force
is 3000#:
   





We have all three sides of the triangle with the angle .
So we can use the law of cosines to find angle 







Since  and  are supplementary,



Edwin